Implement event format V2

Co-Authored-By: erikjohnston <erikj@jki.re>

Check consent dir path on startup

Don't recommend :8448 to people on public_baseurl

Refactor event building into EventBuilder

Fixup calls to `comput_event_signature`

I got fed up with always adding '@unittest.DEBUG' every time I needed to debug a test.

We add the constant, but don't add it to the known room versions. This
lets us start adding V3 logic, but the servers will never join or create
V3 rooms

We currently pass FrozenEvent instead of `dict` to
`compute_event_signature`, which works by accident due to `dict(event)`
producing the correct result.

This fixes PR #4493 commit 855a1510

Implement fallback for V2 invite API

`.user_id` is proxed to `.sender` in FrozenEvent, so this has no
functional change

This is so that everything is done in one place, making it easier to
change the event format based on room version

Split up event validation between event and builder