Fix #3696: API: handle boolean params consistently

* Use ApplicationRecord#attribute_matches to handle boolean attributes
  consistently in search methods.

* Add support for searching various boolean attributes that previously
  weren't supported.

app/models/favorite_group.rb

@@ -65,6 +65,8 @@ class FavoriteGroup < ApplicationRecord
         q = q.name_matches(params[:name_matches])
       end
 
+      q = q.attribute_matches(:is_public, params[:is_public])
+
       q.apply_default_order(params)
     end
   end