Add do_action to groups loop item entries

[hnla]

Groups loop item entries could do with a do_action('bp_before_group_loop_entry')

This was highlighted while tracking down issues where a plugin had attempted to filter markup strings to 'bp_group_is_member' resulting in a car crash.

Plugin was attempting to provide additional meta data about a particular group 'type' this descriptive text could have simply been fed to a do_action just after the loop li element.

It may not be massively useful but is cheap to make provision for?

[DJPaul]

Not going to change BP-Legacy for this (old) ticket, I'll leave it to Hugo to decide if it's something to do in the new template pack and if so, make a ticket over on github.