adding hidden basic asp.net core backend (#30)

Base functionality:

    DB
    email sending
    user registration & login
    storing new characters and reading them back

Notes:

    Functionality is "hidden" with no links pointing to it.
    Most pages are scaffolded and not reviewed at all :(
    local Dev db setup is pushed to git
    NO local email setup :(

Kemkas.Web/Areas/Identity/Pages/Account/Manage/ExternalLogins.cshtml.cs

@@ -0,0 +1,142 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+#nullable disable
+
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading;
+using System.Threading.Tasks;
+using Kemkas.Web.Db.Models;
+using Microsoft.AspNetCore.Authentication;
+using Microsoft.AspNetCore.Identity;
+using Microsoft.AspNetCore.Mvc;
+using Microsoft.AspNetCore.Mvc.RazorPages;
+
+namespace Kemkas.Web.Areas.Identity.Pages.Account.Manage
+{
+    public class ExternalLoginsModel : PageModel
+    {
+        private readonly UserManager<ApplicationUser> _userManager;
+        private readonly SignInManager<ApplicationUser> _signInManager;
+        private readonly IUserStore<ApplicationUser> _userStore;
+
+        public ExternalLoginsModel(
+            UserManager<ApplicationUser> userManager,
+            SignInManager<ApplicationUser> signInManager,
+            IUserStore<ApplicationUser> userStore)
+        {
+            _userManager = userManager;
+            _signInManager = signInManager;
+            _userStore = userStore;
+        }
+
+        /// <summary>
+        ///     This API supports the ASP.NET Core Identity default UI infrastructure and is not intended to be used
+        ///     directly from your code. This API may change or be removed in future releases.
+        /// </summary>
+        public IList<UserLoginInfo> CurrentLogins { get; set; }
+
+        /// <summary>
+        ///     This API supports the ASP.NET Core Identity default UI infrastructure and is not intended to be used
+        ///     directly from your code. This API may change or be removed in future releases.
+        /// </summary>
+        public IList<AuthenticationScheme> OtherLogins { get; set; }
+
+        /// <summary>
+        ///     This API supports the ASP.NET Core Identity default UI infrastructure and is not intended to be used
+        ///     directly from your code. This API may change or be removed in future releases.
+        /// </summary>
+        public bool ShowRemoveButton { get; set; }
+
+        /// <summary>
+        ///     This API supports the ASP.NET Core Identity default UI infrastructure and is not intended to be used
+        ///     directly from your code. This API may change or be removed in future releases.
+        /// </summary>
+        [TempData]
+        public string StatusMessage { get; set; }
+
+        public async Task<IActionResult> OnGetAsync()
+        {
+            var user = await _userManager.GetUserAsync(User);
+            if (user == null)
+            {
+                return NotFound($"Unable to load user with ID '{_userManager.GetUserId(User)}'.");
+            }
+
+            CurrentLogins = await _userManager.GetLoginsAsync(user);
+            OtherLogins = (await _signInManager.GetExternalAuthenticationSchemesAsync())
+                .Where(auth => CurrentLogins.All(ul => auth.Name != ul.LoginProvider))
+                .ToList();
+
+            string passwordHash = null;
+            if (_userStore is IUserPasswordStore<ApplicationUser> userPasswordStore)
+            {
+                passwordHash = await userPasswordStore.GetPasswordHashAsync(user, HttpContext.RequestAborted);
+            }
+
+            ShowRemoveButton = passwordHash != null || CurrentLogins.Count > 1;
+            return Page();
+        }
+
+        public async Task<IActionResult> OnPostRemoveLoginAsync(string loginProvider, string providerKey)
+        {
+            var user = await _userManager.GetUserAsync(User);
+            if (user == null)
+            {
+                return NotFound($"Unable to load user with ID '{_userManager.GetUserId(User)}'.");
+            }
+
+            var result = await _userManager.RemoveLoginAsync(user, loginProvider, providerKey);
+            if (!result.Succeeded)
+            {
+                StatusMessage = "The external login was not removed.";
+                return RedirectToPage();
+            }
+
+            await _signInManager.RefreshSignInAsync(user);
+            StatusMessage = "The external login was removed.";
+            return RedirectToPage();
+        }
+
+        public async Task<IActionResult> OnPostLinkLoginAsync(string provider)
+        {
+            // Clear the existing external cookie to ensure a clean login process
+            await HttpContext.SignOutAsync(IdentityConstants.ExternalScheme);
+
+            // Request a redirect to the external login provider to link a login for the current user
+            var redirectUrl = Url.Page("./ExternalLogins", pageHandler: "LinkLoginCallback");
+            var properties = _signInManager.ConfigureExternalAuthenticationProperties(provider, redirectUrl, _userManager.GetUserId(User));
+            return new ChallengeResult(provider, properties);
+        }
+
+        public async Task<IActionResult> OnGetLinkLoginCallbackAsync()
+        {
+            var user = await _userManager.GetUserAsync(User);
+            if (user == null)
+            {
+                return NotFound($"Unable to load user with ID '{_userManager.GetUserId(User)}'.");
+            }
+
+            var userId = await _userManager.GetUserIdAsync(user);
+            var info = await _signInManager.GetExternalLoginInfoAsync(userId);
+            if (info == null)
+            {
+                throw new InvalidOperationException($"Unexpected error occurred loading external login info.");
+            }
+
+            var result = await _userManager.AddLoginAsync(user, info);
+            if (!result.Succeeded)
+            {
+                StatusMessage = "The external login was not added. External logins can only be associated with one account.";
+                return RedirectToPage();
+            }
+
+            // Clear the existing external cookie to ensure a clean login process
+            await HttpContext.SignOutAsync(IdentityConstants.ExternalScheme);
+
+            StatusMessage = "The external login was added.";
+            return RedirectToPage();
+        }
+    }
+}