Implement event format V2

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 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

This bug was introduced in PR #4470, commit 678a92cb

* fix obvious problem :|

* changelog

Refactor event signing to work on dicts

two reasons for this. One, it saves a bunch of boilerplate. Two, it squashes
unicode to IDNA-in-a-`str` (even on python 3) in a way that it turns out we
rely on to give consistent behaviour between python 2 and 3.

Pass through room version to event auth

If the room version is either 1 or 2 then a server should retry failed
`/v2/invite` requests with the v1 API

* load cert

* changelog

* fix