[#1639] Fix bug when loading the users from ckan.hide_activity_from_users

The problem was that we did set ```user_list```, but never used it.
Merge pull request #1639 from ckan/1639-fix-bug-in-activity-stream-get-filtered-users

_activity_stream_get_filtered_users() doesn't work when there's the config ckan.hide_activity_from_users