[Fix bug 969063][Fix bug 968988] Fix group membership admin for unvouche...

TESTS STILL NEEDED

...d members

autocomplete_light was raising validation errors because its
autocomplete field for members on the groupmembership model
was limited to vouched members but some groups have unvouched
members.

Modified the admin for group memberships to use an autocomplete
field that accepts any user profile.

TESTS STILL NEEDED - I haven't been able to get a test working,
but wanted to submit the fix for review.