OAuth client registration issues

[wmurphyrd]

If a client with the same id is already registered, immers server returns 500 whereas it should return 409

Need a process to remove and replace a client. If a remote server resets their DB that domain name would never be able to connect with this immer again.

1. Require proof of domain ownership (can use http-signaure like in Mastodon secure mode request verification)
2. Revoke any access granted to the previous client