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/api.rs

@@ -108,7 +108,7 @@ pub async fn validate_api_token<B>(req: Request<B>, next: Next<B>) -> impl IntoR
 
 #[derive(Debug, Deserialize)]
 struct AuthenticatedUserParams {
-    github_user_id: Option<i32>,
+    github_user_id: i32,
     github_login: String,
     github_email: Option<String>,
     github_user_created_at: Option<chrono::DateTime<chrono::Utc>>,