This commit isolates the celery task invocations of the userdbs app into signal handlers. All celery interaction is now asynchronously handled in userdbs.signals.
* add userdbs app docstring * add userdbs.app.UserdbsAppConfig * implement userdbs.admin.DatabaseUserCreationForm, userdbs.admin.UserDatabaseCreationForm, userdbs.admin.DatabaseUserAdmin, userdbs.admin.UserDatabaseAdmin * add docstrings to userdbs.models * rename userdbs.models.DatabaseUserManager._get_next_username to _get_next_dbuser_name * fix format string issues in userdbs.models.DatabaseUserManager and userdbs.UserDatabaseManager._get_next_dbname * delete related databases in userdbs.models.UserDatabase.delete
