search children entities

Some entities are organized as a simple tree, we need to make sure that searching for a parent will give results for the children too. If someone searches for "stone", "limestone", "lapis lazuli", and all other materials with a parent of stone, or a parent whose parent is stone, should be picked up.

• Find out how to modify the search queries to take this into account
• Apply it to the following entities: Genres, Materials, Languages