Database schema improvements

Now that we have migrations available, we can make some improvement/updates to the DB schema.

- [x] Slack IDs should be UNIQUE
- [x] Slack usernames aren't always unique in some edge cases - how to handle?
- [ ] Audit which things are nullable - maybe run a migration to fill in some values and make columns NOT NULL where appropriate?

Review the existing migration history and database schema definitions first. Audit nullable columns and identify values that can be backfilled before making columns NOT NULL; done means the schema constraints are justified and the migration applies cleanly without invalid existing data.

Written by the indexing model from the issue text.