Don't redirect to POST request URLs after sign out

Sometimes we define URLs for POST requests which are not defined for GET
requests, such as "/residence", so redirecting to it after signing out
results in a routing error.

So instead of using the request referer, we're using the stored location
devise uses, and we're not storing locations in POST requests.

app/controllers/application_controller.rb

@@ -107,7 +107,7 @@ class ApplicationController < ActionController::Base
     end
 
     def set_return_url
-      if !devise_controller? && is_navigational_format?
+      if request.get? && !devise_controller? && is_navigational_format?
         store_location_for(:user, request.fullpath)
       end
     end