Gateway domain authorisation

[gsvarovsky]

Sharing a domain to more than just the account owner.

Enter principals into the (sub)domain & manage access checks:

• [ ] Ably key capabilities
• [x] Socket.io connection authorisation

Note: this is not necessarily the same concept as 'projects' in timeld.

[gsvarovsky]

• Part of https://github.com/m-ld/sustainable-web-apps/issues/7