Fix wrong redirect after sign-in for RS OAuth

We use a custom auth method to pre-fill the username when reaching the
RS OAuth while signed out. However, it needs to redirect back to the RS
OAuth page after sign-in, and not to the root path.

app/controllers/application_controller.rb

@@ -37,4 +37,8 @@ class ApplicationController < ActionController::Base
       format.any  { head status }
     end
   end
+
+  def after_sign_in_path_for(user)
+    session[:user_return_to] || root_path
+  end
 end