got rid of 'user_filters', moving towards putting filter-clause generators in the...

got rid of 'user_filters', moving towards putting filter-clause generators in the model.

So, if you want a clause that would show only Items accessible by the current user:

  accessible = Item.objects.filter(Item.filter_for_user(current_user))

filter_for_user() returns a Q() object, which can be combined using
binary ops to construct more complex queries.

git-svn-id: svn://svn.open-ils.org/ILS-Contrib/servres/trunk@920 6d9bc8c9-1ec2-4278-b937-99fde70a366f