Login: Add notice about disabled login and hide buttons

public/css/theme-bthree-dark.css

@@ -1,3 +1,7 @@
+/* */
+#navbar .right.stackable.menu {
+  display: none;
+}
 
 /* bthree dark theme */
 @font-face {

templates/user/auth/signin.tmpl

@@ -0,0 +1,25 @@
+{{template "base/head" .}}
+
+<!-- Temporary login code. Remove once projects.blender.org is public. -->
+<div class="page-content user signin">
+	<div class="ui middle very relaxed page grid">
+		<div class="ui container column fluid">
+			<div class="ui warning message">
+				<strong>Login is temporarily disabled.</strong>
+				<br/>It will be enabled again once the migration to projects.blender.org is complete.
+				<a href="https://devtalk.blender.org/">Read more</a>.
+			</div>
+		</div>
+	</div>
+</div>
+<!-- End of temp login code. -->
+
+<div style="display: none;" class="page-content user signin{{if .LinkAccountMode}} icon{{end}}">
+	{{template "user/auth/signin_navbar" .}}
+	<div class="ui middle very relaxed page grid">
+		<div class="ui container column fluid">
+			{{template "user/auth/signin_inner" .}}
+		</div>
+	</div>
+</div>
+{{template "base/footer" .}}