hosted projects (#8627)

- **Allow joining a hosted project**

You can't yet do anything in a hosted project, but you can join it and
look how empty it is.

Release Notes:

- N/A

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

@@ -1,4 +1,4 @@
-use crate::db::{ProjectId, Result, RoomId, ServerId, UserId};
+use crate::db::{HostedProjectId, ProjectId, Result, RoomId, ServerId, UserId};
 use anyhow::anyhow;
 use rpc::ConnectionId;
 use sea_orm::entity::prelude::*;
@@ -8,10 +8,11 @@ use sea_orm::entity::prelude::*;
 pub struct Model {
     #[sea_orm(primary_key)]
     pub id: ProjectId,
-    pub room_id: RoomId,
-    pub host_user_id: UserId,
+    pub room_id: Option<RoomId>,
+    pub host_user_id: Option<UserId>,
     pub host_connection_id: Option<i32>,
     pub host_connection_server_id: Option<ServerId>,
+    pub hosted_project_id: Option<HostedProjectId>,
 }
 
 impl Model {