Allow nulls in projects.host_connection_{id,server_id}

The server version on stable won't be able to fill values for those
columns when we deploy the migration to preview.

With this commit we're also dropping the unused `worktree_extensions`
and `project_activity_periods` tables. The last version of the server
on stable (0.2.6) doesn't contain any code that accesses those tables.

crates/collab/src/rpc.rs

@@ -1506,7 +1506,6 @@ async fn update_buffer(
         .project_connection_ids(project_id, session.connection_id)
         .await?;
 
-    dbg!(session.connection_id, &*project_connection_ids);
     broadcast(
         session.connection_id,
         project_connection_ids.iter().copied(),