Fixes for email filtering

Reviewed By: Peter Rotich 07/22/12

Canned Autoresponse

Reviewed By: Peter Rotich 07/22/12

Allow members of a group access to a department

Reviewed-by: Jared Hancock <jared@osticket.com>

And remove unsupported filter of email headers

Change enableGroupMembership() to isGroupMembershipEnabled(). Improve getMembers function. Make sure  is an array in updateAllowedGroups

But don't send out canned responses for new tickets that matched a filter
with an attached canned response if that response is not enabled. Also,
explicitly block deletes to filters that are in use.