Adds filters to search form HTML functions

Filters introduced in bp_directory_members_search_form(),
bp_directory_groups_search_form(), and bp_group_search_form() allow theme
and plugin authors more flexibility with core-generated search form HTML.

Fixes #4974

Props dimensionmedia

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7328 cdf35c40-ae34-48e0-9cc9-0c9da1808c22