Use WP Plugin API rather than call_user_func() to apply format_callback to activity actions

This allows plugin authors to modify or override default formatting callbacks
using the standard remove_filter() and add_filter() API functions.

Hat-tip to nacin for the idea

See #3856

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8126 cdf35c40-ae34-48e0-9cc9-0c9da1808c22
Pinking shears

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8127 cdf35c40-ae34-48e0-9cc9-0c9da1808c22