Make sure we advertise registration as disabled when MSC3861 is enabled

[sandhose]

This has been a problem with Element Web, as it will proble /register with an empty body, which gave this error:

curl -d '{}' -HContent-Type:application/json /_matrix/client/v3/register

{"errcode": "M_UNKNOWN",
 "error": "Invalid username"}

And Element Web would choke on it. This changes that so we reply instead:

{"errcode": "M_FORBIDDEN",
 "error": "Registration has been disabled. Only m.login.application_service registrations are allowed."}

Also adds a test for this.

See element-hq/element-web#27993

[anoadragon453]

The code change looks fine. Could you point to where in the MSCs it says this endpoint should return an errcode of M_FORBIDDEN?

[anoadragon453]

Do we have a test that checks that this endpoint still returns a 200 when an AS attempts to register a user?