* Move common methods from limitmonitor/task_resources/subscription.py
  to limitmonitor/common.py
* Add disable() and is_enabled() methods for Limit class.
* Rise exception when we cannot terminate a Tunnel account.
* Add get_enabled_limits() method for User class.
* Remove redundant code in process_pending_registrations_user.
* Use limit.is_enabled() instead of limit.is_active.

Signed-off-by: Birin Sanchez <birin.sanchez@puri.sm>