Fixes #5905

Enable cleaning up extremities with dummy events by default to prevent undue build up of forward extremities. (#5884)

Fixes #6103

If email or msisdn verification aren't supported, let's stop advertising them
for registration.

Fixes #6100.

Pull the checkers out to their own classes, rather than having them lost in a
massive 1000-line class which does everything.

This is also preparation for some more intelligent advertising of flows, as per #6100

because, frankly, it looked like it was written by an axe-murderer.

This should be a non-functional change, except that where `m.login.dummy` was
previously advertised *before* `m.login.terms`, it will now be advertised
afterwards. AFAICT that should have no effect, and will be more consistent with
the flows that involve passing a 3pid.

Added in #5377, apparently in error

Second part of solving #6076
Fixes #6076

We return a submit_url parameter on calls to POST */msisdn/requestToken so that clients know where to submit token information to.

Add a m.separate_add_and_bind flag set to True. See MSC2290's Backward Compatibility section for details.

Uses a SimpleHttpClient instance equipped with the federation_ip_range_blacklist list for requests to identity servers provided by user input. Does not use a blacklist when contacting identity servers specified by account_threepid_delegates. The homeserver trusts the latter and we don't want to prevent homeserver admins from specifying delegates that are on internal IP addresses.

Fixes #5935

Adds a flag to /versions' unstable_features section indicating that this Synapse understands what an id_access_token is, as per MSC2264.

Fixes #5927

As MSC2263 states, m.require_identity_server must be set to false when it does not require an identity server to be provided by the client for the purposes of email registration or password reset.

Adds an m.require_identity_server flag to /versionss unstable_flags section. This will advertise that Synapse no longer needs id_server as a parameter.

First part of solving #6076

Implements MSC2290. This PR adds two new endpoints, /unstable/account/3pid/add and /unstable/account/3pid/bind. Depending on the progress of that MSC the unstable prefix may go away.

This PR also removes the blacklist on some 3PID tests which occurs in #6042, as the corresponding Sytest PR changes them to use the new endpoints.

Finally, it also modifies the account deactivation code such that it doesn't just try to deactivate 3PIDs that were bound to the user's account, but any 3PIDs that were bound through the homeserver on that user's account.

Fixes #6066

This register endpoint should be disabled if registration is disabled, otherwise we're giving anyone the ability to check if a username exists on a server when we don't need to be.

Error code is 403 (Forbidden) as that's the same returned by /register when registration is disabled.

In ancient times Synapse would only send emails when it was notifying a user about a message they received...

Now it can do all sorts of neat things!

Change the logging so it's not just about notifications.

Remove trailing slash ability from the password reset submit_token endpoint. Since we provide the link in an email, and have never sent it with a trailing slash, there's no point for us to accept them on the endpoint.

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.

Fixes a bug where the default attribute maps were prioritised over
user-specified ones, resulting in incorrect mappings.

The problem is that if you call SPConfig.load() multiple times, it adds new
attribute mappers to a list. So by calling it with the default config first,
and then the user-specified config, we would always get the default mappers
before the user-specified mappers.

To solve this, let's merge the config dicts first, and then pass them to
SPConfig.

 * 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.