Undo making project optional on stored follower states

Following works without a project, but following in unshared projects does
not need to be replicated to other participants.

crates/collab/src/db/tables/follower.rs

@@ -8,7 +8,7 @@ pub struct Model {
     #[sea_orm(primary_key)]
     pub id: FollowerId,
     pub room_id: RoomId,
-    pub project_id: Option<ProjectId>,
+    pub project_id: ProjectId,
     pub leader_connection_server_id: ServerId,
     pub leader_connection_id: i32,
     pub follower_connection_server_id: ServerId,