collab: Make users.github_user_id required and unique (#16704)

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

crates/collab/migrations/20240822215737_add_unique_constraint_on_github_user_id_on_users.sql

@@ -0,0 +1,4 @@
+alter table users alter column github_user_id set not null;
+
+drop index index_users_on_github_user_id;
+create unique index uix_users_on_github_user_id on users (github_user_id);