Decouple project deletion from thread deletion (#4673)

* Decouple project deletion from thread deletion

* Allow a thread to exist without a project

* attempt 2

* Modify migration to set orphaned threads' mods to NULL instead of removing constraint entirely

* Use mod PAT for mod threads

apps/labrinth/src/database/models/project_item.rs

@@ -384,9 +384,6 @@ impl DBProject {
             .execute(&mut **transaction)
             .await?;
 
-            models::DBThread::remove_full(project.thread_id, transaction)
-                .await?;
-
             sqlx::query!(
                 "
                 UPDATE reports