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

app/controllers/rs/oauth_controller.rb

@@ -108,6 +108,7 @@ class Rs::OauthController < ApplicationController
   def require_signed_in_with_username
     unless user_signed_in?
       username, org = params[:useraddress].split("@")
+      session[:user_return_to] = request.url
       redirect_to new_user_session_path(cn: username, ou: org)
     end
   end