This PR makes the `github_user_created_at` field required at ingress
points into collab.
In practice we already have this value passed up, this change just makes
that explicit.
This is a precursor to making it required in the database.
Release Notes:
- N/A
This PR makes the `github_user_id` column on the `users` table required
and replaces the index with a unique index.
I have gone through and ensured that all users have a unique
`github_user_id` in the staging and production databases.
Release Notes:
- N/A
