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/tests/channel_guest_tests.rs

@@ -168,7 +168,7 @@ async fn test_channel_requires_zed_cla(cx_a: &mut TestAppContext, cx_b: &mut Tes
     server
         .app_state
         .db
-        .get_or_create_user_by_github_account("user_b", Some(100), None, Some(Utc::now()), None)
+        .get_or_create_user_by_github_account("user_b", 100, None, Some(Utc::now()), None)
         .await
         .unwrap();
 
@@ -266,7 +266,7 @@ async fn test_channel_requires_zed_cla(cx_a: &mut TestAppContext, cx_b: &mut Tes
     server
         .app_state
         .db
-        .add_contributor("user_b", Some(100), None, Some(Utc::now()), None)
+        .add_contributor("user_b", 100, None, Some(Utc::now()), None)
         .await
         .unwrap();