Rate-limit outgoing read-receipts as per #4730.

turns out this relies on there being a `user_consent_version` set.

Rather than using a Mock for the homeserver config, use a genuine
HomeServerConfig object. This makes for a more realistic test, and means that
we don't have to keep remembering to add things to the mock config every time
we add a new config setting.

Make sure that we have a `server_notices_mxid` set, given that we are relying
on it.

* Set allow_guest_access = True, since we rely on it
* config doesn't have a `hostname` attribute; it is `server_name`

The Mailer expects the config object to have `email_smtp_pass` and
`email_riot_base_url` attributes (and it won't by default, because the default
config impl doesn't set any of the attributes unless email_enable_notifs is
set).

Fixes a bug where hs_disabled_message was not enforced for 3pid-based requests
if there was no server_notices_mxid configured.

Add two ratelimiters on login (per-IP address and per-userID).

This reverts commit 290552fd.

Server side of a solution towards #3622.

This reverts commit d7dbad35.

* Rate-limiting for registration

* Add unit test for registration rate limiting

* Add config parameters for rate limiting on auth endpoints

* Doc

* Fix doc of rate limiting function

Co-Authored-By: babolivier <contact@brendanabolivier.com>

* Incorporate review

* Fix config parsing

* Fix linting errors

* Set default config for auth rate limiting

* Fix tests

* Add changelog

* Advance reactor instead of mocked clock

* Move parameters to registration specific config and give them more sensible default values

* Remove unused config options

* Don't mock the rate limiter un MAU tests

* Rename _register_with_store into register_with_store

* Make CI happy

* Remove unused import

* Update sample config

* Fix ratelimiting test for py2

* Add non-guest test

This is required because the 'Server' HTTP header is not always
passed through proxies.

We will later need also to import 'register_servlets' from the
'login' module, so we un-pollute the namespace now to keep the
logical changes separate.

* Fix parsing of Content-Disposition headers

TIL: filenames in content-dispostion headers can contain semicolons, and aren't
%-encoded.

* fix python2 incompatibility

* Fix docstrings

* Move RegistrationHandler init to HomeServer

* Move post registration actions to RegistrationHandler

* Add post regisration replication endpoint

* Newsfile

This allows specifying who and what is allowed to be published onto the
public room list