Introduce basic community visibility feature.

Site admins now have the option to restrict access to the
community portion of a BuddyPress site. If the site admin
chooses "members only," then, when a non-logged-in
user attempts to visit a BuddyPress screen, she will
be shown an access error message and a log-in form.

Props imath, sbrajesh, jjj, shawfactor.

Fixes #8734.