Improved management of new_blog_post activity items at transition_post_status

Introduces bp_blogs_catch_transition_post_status(). This function, which
replaces bp_blogs_catch_published_post(), performs a similar task - determining
when a new blog post has been published, and recording the post in the activity
stream - but adds a corrolary for unpublishing that deletes the activity item.

Unit tests and deprecated function are included.

See #5333

Props henry.wright for an initial patch

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8073 cdf35c40-ae34-48e0-9cc9-0c9da1808c22