Add render_if_exists to inject additional search field in EE

This allows EE code to reuse the search partial and append additional
query parameters to it, and have it preserved when searching.

app/views/shared/projects/_search_form.html.haml

@@ -21,3 +21,5 @@
 
   - if params[:visibility_level].present?
     = hidden_field_tag :visibility_level, params[:visibility_level]
+
+  = render_if_exists 'shared/projects/search_fields'