GRN2-xx: Switch the relation between users and roles to make queries cleaner and faster (#1299)

* First steps

* Fixes in account creation flow

* Fixed most testcases

* more test fixes

* Fixed more test cases

* Passing tests and rubocop

* Added rake task to remove rooms

app/controllers/application_controller.rb

@@ -26,7 +26,7 @@ class ApplicationController < ActionController::Base
 
   # Retrieves the current user.
   def current_user
-    @current_user ||= User.includes(:roles, :main_room).find_by(id: session[:user_id])
+    @current_user ||= User.includes(:role, :main_room).find_by(id: session[:user_id])
 
     if Rails.configuration.loadbalanced_configuration
       if @current_user && !@current_user.has_role?(:super_admin) &&