🐞🔨Authentication Prevent a condition when double-tapping sign-in

- #1809

OK, so my working assumption is that the bug is caused by two requests
being processed at the same time for a new email address, and the first
one creates the `AuthenticationMethod` and the second one *tries* to,
and fails; resulting in the validation error.

This fixes the racecondition at the controller level, but I think we
want to do it at in AuthenticatedSession#save instead...

app/controllers/authenticated_sessions_controller.rb

@@ -15,6 +15,13 @@ def create
     else
       render :create, status: :unprocessable_entity
     end
+  rescue ActiveRecord::RecordInvalid
+    @authenticated_session = nil
+    if authenticated_session.save
+      redirect_to(current_space.presence || :root)
+    else
+      render :create, status: :unprocessable_entity
+    end
   end
 
   alias_method :update, :create