Pass the group id to the edit_ and create_ methods of BP_Group_Extension

This change creates parity with the admin_screen() and admin_screen_save()
methods, which will make it easier for plugin authors to abstract shared logic.

See #4955

git-svn-id: https://buddypress.svn.wordpress.org/trunk@6992 cdf35c40-ae34-48e0-9cc9-0c9da1808c22