Set initial group metadata in groups_create_group() rather than action function

Groups created through the user interface have two pieces of metadata set in
groups_action_create_group(): the total_member_count and the last_activity.
(Both items are needed for newly created groups to show up in directories.)
However, the placement of these groups_update_groupmeta() calls - in what is
essentially a controller function - meant that groups created programatically
using groups_create_group() did not have their complete metadata set. Moving
the metadata setting into groups_create_group() means one less step for plugins
and other scripts that create groups programatically.

Fixes #4827

Props ericlewis, rogercoathup

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7172 cdf35c40-ae34-48e0-9cc9-0c9da1808c22