Make sure BP_User_Query returns correct results when search term contains the Ampersand character

1. Search terms containing this character was problematic as the "&" is a query var delimiter used in {{{bp_legacy_theme_ajax_querystring()}}} to build the ajax querystring

2. When a xProfile field is saved, the value is sanitized using the {{{xprofile_filter_kses()}}} filter which is converting "&" to "&amp;" before inserting the value in database.

Urlencoding the search terms in {{{bp_legacy_theme_ajax_querystring()}}} and applying {{{wp_kses_normalize_entities()}}} to search terms in {{{BP_User_Query}}} & in {{{bp_xprofile_bp_user_query_search()}}} make sure the correct results are returned to the user.

Fixes #5694

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8928 cdf35c40-ae34-48e0-9cc9-0c9da1808c22