Add two new filters to allow group order to be customized in Groups directory

The first filter {{{bp_groups_get_orderby}}} will help to customize the order and orderby attributes of {{{BP_Groups_Group::get()}}} method.
The second one {{{bp_groups_get_orderby_converted_by_term}}} will help to edit  the corresponding SQL term/column.

Fixes #3161

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8669 cdf35c40-ae34-48e0-9cc9-0c9da1808c22