In bp_activity_add_user_favorite(), use $user_id param for usermeta

The function bp_activity_add_user_favorite() accepts a $user_id parameter, but
previously the parameter was not actually used to retrieve and set the relevant
usermeta in the function (it was hardcoded to use the loggedin user id). This
changeset corrects that oversight.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@6925 cdf35c40-ae34-48e0-9cc9-0c9da1808c22
In bp_has_activities(), allow 'favorites' to be filtered by 'Show' dropdown

Switching the 'favorites' logic to interpret the favorite ids as part of the
'in' parameter rather than the 'include' paramater allows other filter params,
including those in the 'Show' dropdown on activity directories (the 'action'
parameter of bp_has_activities()) to work.

Fixes #4872

Props chroniko

git-svn-id: https://buddypress.svn.wordpress.org/trunk@6926 cdf35c40-ae34-48e0-9cc9-0c9da1808c22