Adding 2FA Authenticator Support (#66)

* Adding support for 2fa, more auth options

* WIP getting auth stuff working

* Added Manage views. 2FA working now for MVC app.

* Switching to using a controller for no-UI logout scenario

* Adding Razor Pages impl of 2FA auth stuff. Works.

src/Web/Views/Manage/Disable2fa.cshtml

@@ -0,0 +1,24 @@
+@{
+    ViewData["Title"] = "Disable two-factor authentication (2FA)";
+    ViewData.AddActivePage(ManageNavPages.TwoFactorAuthentication);
+}
+
+<h2>@ViewData["Title"]</h2>
+
+<div class="alert alert-warning" role="alert">
+    <p>
+        <span class="glyphicon glyphicon-warning-sign"></span>
+        <strong>This action only disables 2FA.</strong>
+    </p>
+    <p>
+        Disabling 2FA does not change the keys used in authenticator apps. If you wish to change the key
+        used in an authenticator app you should <a asp-action="ResetAuthenticatorWarning">reset your
+        authenticator keys.</a>
+    </p>
+</div>
+
+<div>
+    <form asp-action="Disable2fa" method="post" class="form-group">
+        <button class="btn btn-danger" type="submit">Disable 2FA</button>
+    </form>
+</div>