Show correct number of members in Group

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 in http://manage.dimagi.com/default.asp?134040  This
commit uses the filtered list of users already retrieved and memoized in
the view instead.
Use iter-docs and don't keep passing through the list
Wrap CouchUser correctly
Merge pull request #4319 from dimagi/group_membership

Show correct number of members in Group