Fall back on displayed user id in some activity template functions

In bp_get_total_favorite_count_for_user() and
bp_get_total_mention_count_for_user(), it's convenient to default to the
displayed user id when no id is passed manually.

Fixes #4998

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7129 cdf35c40-ae34-48e0-9cc9-0c9da1808c22