backfill slugs on existing synonym records

[Courey]

When we changed the route to be a resource/id route to use a slug, I had to add a field to the synonyms table called slug. We need to run through all the existing synonym records and add the slugified version of the eventId to the slug field. The code in review in PR #2617 will not function as expected until we add the slug to the existing records.
use import { slug } from 'github-slugger' to create the slugs.