Fixes:
> TypeError: set_destination_retry_timings() missing 1 required positional
argument: 'retry_interval'

Introduced in #6016.

The validation links sent via email had their query parameters inserted without any URL-encoding. Surprisingly this didn't seem to cause any issues, but if a user were to put a `/` in their client_secret it could lead to problems.

As per [MSC2290](https://github.com/matrix-org/matrix-doc/pull/2290/files#diff-05cde9463e9209b701312b3baf2fb2ebR151), we're dropping the bind parameter from `/account/3pid`. This endpoint can now only be used for adding threepid's to the user's account on the homeserver.

* Allow passing SYNAPSE_WORKER envvar

* changelog.d

* Document SYNAPSE_WORKER.

Attempting to imply that you don't need to change this default
unless you're in worker mode.

Also aware that there's a bigger problem of attempting to document
a complete working configuration of workers using docker, as we
currently only document to use `synctl` for worker mode, and synctl
doesn't work that way in docker.

 * make it clear that if you installed from a package manager, you should use
   that to upgrade

 * Document the new way of getting the server version (cf #4878)

 * Write some words about downgrading.

This is a partial revert of #5893. The problem is that if we drop these tables
in the same release as removing the code that writes to them, it prevents users
users from being able to roll back to a previous release.

So let's leave the tables in place for now, and remember to drop them in a
subsequent release.

(Note that these tables haven't been *read* for *years*, so any missing rows
resulting from a temporary upgrade to vNext won't cause a problem.)

Removes the POST method from `/password_reset/<medium>/submit_token/` as it's only used by phone number verification which Synapse does not support yet.

This checks whether the current platform supports thread resource usage tracking
before logging a warning to avoid log spam.

Fixes https://github.com/matrix-org/synapse/issues/6055

this was apparently broken by #6040.

3PID invites require making a request to an identity server to check that the invited 3PID has an Matrix ID linked, and if so, what it is.

These requests are being made on behalf of a user. The user will supply an identity server and an access token for that identity server. The homeserver will then forward this request with the access token (using an `Authorization` header) and, if the given identity server doesn't support v2 endpoints, will fall back to v1 (which doesn't require any access tokens).

Requires: ~~#5976~~

Converting some of the rst documentation to markdown.  Attempted to
preserve whitespace and line breaks to minimize cosmetic change.

Broke in #5971

Basically the bug is that if get_current_state_deltas returns no new updates and we then take the max pos, its possible that we miss an update that happens in between the two calls. (e.g. get_current_state_deltas looks up to stream pos 5, then an event persists and so getting the max stream pos returns 6, meaning that next time we check for things with a stream pos bigger than 6)

Track the time that a server started failing at, for general analysis purposes.

Essentially the intention here is to end up blacklisting servers which never
respond to federation requests.

Fixes https://github.com/matrix-org/synapse/issues/5113.

This was intended to introduce an element of jitter; instead it gave you a
30/60 chance of resetting to zero.

Fix SSO fallback login

Well, it worked, but forgot to remove the thing saying login was
unavailable.

This PR adds the optional `report_stats_endpoint` to configure where stats are reported to, if enabled.

* Blow up config if opentracing is missing

Ensure support users can be registered even if MAU limit is reached

This allows support users to be created even on MAU limits via
the admin API. Support users are excluded from MAU after creation,
so it makes sense to exclude them in creation - except if the
whole host is in disabled state.

Signed-off-by: Jason Robinson <jasonr@matrix.org>

This is a redo of https://github.com/matrix-org/synapse/pull/5897 but with `id_access_token` accepted.

Implements [MSC2134](https://github.com/matrix-org/matrix-doc/pull/2134) plus Identity Service v2 authentication ala [MSC2140](https://github.com/matrix-org/matrix-doc/pull/2140).

Identity lookup-related functions were also moved from `RoomMemberHandler` to `IdentityHandler`.

Allow use of different ratelimits for admin redactions.

* remove some unused code
* make things which were constants into constants for efficiency and clarity

Some small fixes to `room_member.py` found while doing other PRs.

1. Add requester to the base `_remote_reject_invite` method.
2. `send_membership_event`'s docstring was out of date and took in a `remote_room_hosts` arg that was not used and no calling function provided. 

Co-Authored-By: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>

Two things I missed while implementing [MSC2140](https://github.com/matrix-org/matrix-doc/pull/2140/files#diff-c03a26de5ac40fb532de19cb7fc2aaf7R80).

1. Access tokens should be provided to the identity server as `access_token`, not `id_access_token`, even though the homeserver may accept the tokens as `id_access_token`.
2. Access tokens must be sent to the identity server in a query parameter, the JSON body is not allowed.

We now send the access token as part of an `Authorization: ...` header, which fixes both things.

The breaking code was added in https://github.com/matrix-org/synapse/pull/5892

Sytest PR: https://github.com/matrix-org/sytest/pull/697