Bug 877315 - count vouched members in admin

In the admin list for groups, add a column to display the number of
vouched members, similar to the current column that counts all
members.

This implementation will not be very efficient, as it'll generate
another query for every group on the page. I don't think we can
do better using built-in Django functionality for annotating queries,
the way the column counting all members works, though it might be
possible to add some raw SQL to that query to do it. This might be
good enough, though.