Show each person only the projects they can access #509

Part of #506. Depends on the access check.

Each host shows its owner only projects the owner can access:

• Filter the directory's listings before the app sees them.
• Subscribe only to lobbies of projects the owner can access, and drop lobby events for projects without access.
• Hide hosted channels in projects without access, and refuse to relay requests to them.
• When access is lost, the project and its channels disappear from the sidebar and project picker, along with presence.