REGISTRAR: catch other exceptions too when auto-approving

- When auto-approving application, catch also UserExtSourceNotExists
  and ExtSourceNotExists exceptions, since it means, that user is unknown
  and we can't allow to approve Group initial application.
GUI: added more available tag options to help

- Added more available tags when creating/editing notifications.
Merge pull request #440 from zlamalp/registrar

REGISTRAR: catch other exceptions too when auto-approving