The old website expects a slides_link field in the talks table. We now store this information in the new talk_links table, but we need to manage the migration here.
When a talk_link is created, add the URL to the talks table if there is not already a URL there.
When a talk_link is edited and the URL changes, check if the same URL was stored in the talk.slides_link column and if so, update that also.
When a talk_link is deleted, if its URL is used in the talks table, empty that field. If there are other valid talk_links for this talk, add one to the talks table.