Show correct number of members in Group

http://manage.dimagi.com/default.asp?134040
The Group Membership page used to use group.users, which is a list of all user ids for that group.  This includes deleted users, which was causing the issue. This PR uses the filtered list of users already retrieved and memoized in the view instead.

Second commit switches the get_users call to Use iter-docs and not keep passing through the list