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/migrations/20240227215556_hosted_projects_in_projects.sql

@@ -0,0 +1,3 @@
+-- Add migration script here
+ALTER TABLE projects ALTER COLUMN host_user_id DROP NOT NULL;
+ALTER TABLE projects ADD COLUMN hosted_project_id INTEGER REFERENCES hosted_projects(id) UNIQUE NULL;