bp_has_activities() show default to show_hidden=true for members of a group

Requiring the group to be non-public, as was previously the case, meant that
group members were unable to see legitimate group activity that was marked
hide_sitewide for some reason (such as when a group was previously non-public).

Fixes #2014

Props sbrajesh

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8404 cdf35c40-ae34-48e0-9cc9-0c9da1808c22