Add search support in Ajax "Load More" and "Load Newest" activity actions

If a search is performed in activities using the query string "?s=search_terms", make sure the search terms are sent in the ajax arguments of the "Load More" and the "Load Newest" actions.

Fixes #5779

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8817 cdf35c40-ae34-48e0-9cc9-0c9da1808c22