Avoid PHP notices during unit tests

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8406 cdf35c40-ae34-48e0-9cc9-0c9da1808c22
Introduce 'active_format' param for bp_get_blog_last_active()

This allows themers to skip the 'active %s' wrapper that BP puts around the
date of last activity for a blog.

See #5392

Props lenasterg

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8407 cdf35c40-ae34-48e0-9cc9-0c9da1808c22