Generalize notifications' actor id to entity id

This way, we can retrieve channel invite notifications when
responding to the invites.

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

@@ -9,8 +9,8 @@ pub struct Model {
     pub id: NotificationId,
     pub created_at: PrimitiveDateTime,
     pub recipient_id: UserId,
-    pub actor_id: Option<UserId>,
     pub kind: NotificationKindId,
+    pub entity_id: Option<i32>,
     pub content: String,
     pub is_read: bool,
     pub response: Option<bool>,