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/src/db/tests/buffer_tests.rs

@@ -42,7 +42,7 @@ async fn test_channel_buffers(db: &Arc<Database>) {
             false,
             NewUserParams {
                 github_login: "user_c".into(),
-                github_user_id: 102,
+                github_user_id: 103,
             },
         )
         .await