Match user_nicename and user_login in addition to xprofile fields in messages autocomplete

The AJAX handler bp_legacy_theme_ajax_messages_autocomplete_results() now uses
BP_User_Query and its search_terms parameter for matching users.

Fixes #5155

Props imath for an initial patch

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8088 cdf35c40-ae34-48e0-9cc9-0c9da1808c22