Sound the knell for the BuddyPress Default theme

bp-default is being sunsetted as of BP 1.9. The theme will continue to receive
security updates and other critical fixes, but will otherwise no longer be
under active development by the core team.

We maintain backward compatibility with sites currently using bp-default by
continuing to register the bp-themes theme directory when bp-default, or a
child theme thereof, is in use on a given site. Those sites will continue to
be able to use the theme. Installations where bp-default is not already in use
will no longer see it listed on Dashboard > Appearance.

Site administrators or developers who want to override this behavior may do so
with the 'bp_do_register_theme_directory' filter. Please note that bp-themes
will be removed altogether from BuddyPress in a future version, likely migrated
to wordpress.org/extend. See #5212.

> Now cracks a noble heart. Good-night, sweet prince;
> And flights of angels sing thee to thy rest.

Fixes #5223

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7569 cdf35c40-ae34-48e0-9cc9-0c9da1808c22