Alert profile listing "Remove selected" button needn't be visible when there are no profiles to select.

[lunkwill42]

If a user, who has never had any alert profile, clicks the "Remove selected" button, the resulting page crashes, because the user has never had an AlertPreference entry.

This is the traceback (NAV 4.8.5):

Internal Server Error: /alertprofiles/profile/remove/
Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/django/core/handlers/base.py", line 111, in get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/usr/lib/python2.7/dist-packages/nav/web/alertprofiles/decorators.py", line 46, in _handler
    return func(request, *args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/nav/web/alertprofiles/views.py", line 356, in profile_remove
    account=account).active_profile
  File "/usr/lib/python2.7/dist-packages/django/db/models/manager.py", line 92, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/django/db/models/query.py", line 357, in get
    self.model._meta.object_name)
DoesNotExist: AlertPreference matching query does not exist.