Do not load autoreply forms for admins with no mailbox

[tonioo]

Traceback:
File "/srv/modoboa/env/local/lib/python2.7/site-packages/django/core/handlers/base.py" in get_response
  111.                     response = wrapped_callback(request, *callback_args, **callback_kwargs)
File "/srv/modoboa/env/local/lib/python2.7/site-packages/django/db/transaction.py" in inner
  394.                 return func(*args, **kwargs)
File "/srv/modoboa/env/local/lib/python2.7/site-packages/django/db/transaction.py" in inner
  394.                 return func(*args, **kwargs)
File "/srv/modoboa/env/local/lib/python2.7/site-packages/django/contrib/auth/decorators.py" in _wrapped_view
  21.                 return view_func(request, *args, **kwargs)
File "/srv/modoboa/env/local/lib/python2.7/site-packages/django/contrib/auth/decorators.py" in _wrapped_view
  21.                 return view_func(request, *args, **kwargs)
File "/srv/modoboa/env/local/lib/python2.7/site-packages/reversion/revisions.py" in do_revision_context
  297.                 return func(*args, **kwargs)
File "/srv/modoboa/env/local/lib/python2.7/site-packages/modoboa_admin/views/identity.py" in editaccount
  180.     events.raiseEvent("FillAccountInstances", request.user, account, instances)
File "/srv/modoboa/env/local/lib/python2.7/site-packages/modoboa/lib/events.py" in raiseEvent
  90.         callback(*args, **kwargs)
File "/srv/modoboa/env/local/lib/python2.7/site-packages/modoboa_postfix_autoreply/general_callbacks.py" in fill_account_tab
  55.         if account.mailbox:
File "/srv/modoboa/env/local/lib/python2.7/site-packages/django/db/models/fields/related.py" in __get__
  428.                     self.related.get_accessor_name()

Exception Type: RelatedObjectDoesNotExist at /admin/accounts/edit/1/
Exception Value: User has no mailbox.