New feature: add access_helper.
New feature: add access_helper.
Change in behavior: rename route action accounts#roles to accounts#update_roles, add accounts#update_multiple action to routes.
Code refactoring: refactor auths and sessions controllers with access helper methods.
Change in behavior: deny moderator to edit roles.
Bug fixing: fix the bug in switching account states.
Code refactoring: refactor the AccountsController