BP Nouveau: bring back search form filters

Although it was already possible to override search form templates, as the following search form filters are not Legacy filters but Core components ones, we are bringing them back into the Nouveau template pack:

• bp_directory_members_search_form
• bp_directory_blogs_search_form
• bp_directory_groups_search_form
• bp_message_search_form

Props wbcomdesigns, DJPaul, vapvarun

Fixes #7881 (trunk)