Why

This PR addresses the following problem / context:

• when a thing is deleted its child thing_id_links need to be deleted. If not, the thing_id field is NULL, which breaks the not-null constraint that comes from the foreign key

How

Implementation summary - the following was changed / added / removed:

• add a reference to the thing_id_link table in the thing table and set cascade="all, delete-orphan"

Notes

Any special considerations, workarounds, or follow-up work to note?

• Use bullet points here