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

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

Implement fallback for V2 invite API

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

The validator was being run on the EventBuilder objects, and so the
validator only checked a subset of fields. With the upcoming
EventBuilder refactor even fewer fields will be there to validate.

To get around this we split the validation into those that can be run
against an EventBuilder and those run against a fully fledged event.

This is in preparation for making EventBuilder format agnostic, which
means event signing should be done against the event dict rather than
the EventBuilder object.

Migrate direct message and tag state on room upgrade

* Remove --process-dependency-links from UPGRADE.rst

Signed-off-by: Aaron Raimist <aaron@raim.ist>

* Add changelog

Signed-off-by: Aaron Raimist <aaron@raim.ist>

Refactoring in MatrixFederationAgent

We don't want to be doing .well-known lookups on these guys.