This adds an API for third-party plugin modules to implement account validity, so they can provide this feature instead of Synapse. The module implementing the current behaviour for this feature can be found at https://github.com/matrix-org/synapse-email-account-validity.
To allow for a smooth transition between the current feature and the new module, hooks have been added to the existing account validity endpoints to allow their behaviours to be overridden by a module.
This attempts to be a direct port of https://github.com/matrix-org/synapse-dinsic/pull/74 to mainline. There was some fiddling required to deal with the changes that have been made to mainline since (mainly dealing with the split of `RegistrationWorkerStore` from `RegistrationStore`, and the changes made to `self.make_request` in test code).
Part of #9744
Removes all redundant `# -*- coding: utf-8 -*-` lines from files, as python 3 automatically reads source code as utf-8 now.
`Signed-off-by: Jonathan de Jong <jonathan@automatia.nl>`
This is a combination of a few different PRs, finally all being merged into `develop`:
* #5875
* #5876
* #5868 (This one added the `/versions` flag but the flag itself was actually [backed out](891afb57cb (diff-e591d42d30690ffb79f63bb726200891)) in #5969. What's left is just giving /versions access to the config file, which could be useful in the future)
* #5835
* #5969
* #5940
Clients should not actually use the new registration functionality until https://github.com/matrix-org/synapse/pull/5972 is merged.
UPGRADE.rst, changelog entries and config file changes should all be reviewed closely before this PR is merged.
