Improve the way Group's Manage tabs are generated

In {{{BP_Group_Extension->setup_edit_hooks()}}} method, we are now using the {{{bp_core_new_subnav_item()}}} function to generate the Group's manage tab sub navigation.
The action {{{'groups_admin_tabs'}}} is no more used. However for back compatibility reasons, we will keep on catching this hook inviting the user using a {{{_doing_it_wrong}}} message to now use the BuddyPress Group Extension API instead.

props boonebgorges

Fixes #5994

git-svn-id: https://buddypress.svn.wordpress.org/trunk@9127 cdf35c40-ae34-48e0-9cc9-0c9da1808c22