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/tables/user.rs

@@ -10,7 +10,7 @@ pub struct Model {
     #[sea_orm(primary_key)]
     pub id: UserId,
     pub github_login: String,
-    pub github_user_id: Option<i32>,
+    pub github_user_id: i32,
     pub github_user_created_at: Option<NaiveDateTime>,
     pub email_address: Option<String>,
     pub admin: bool,