add context to phonehome stats

Adds a new method, check_3pid_auth, which gives password providers
the chance to allow authentication with third-party identifiers such
as email or msisdn.

This one should close #4841. Many thanks to @dev4223 for bringing it up and finding a solution.

Signed-off-by: Colin White

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

This disables both local and remote room list searching.

Make it so that most options in the config are optional, and commented out in
the generated config.

The reasons this is a good thing are as follows:

* If we decide that we should change the default for an option, we can do so,
  and only those admins that have deliberately chosen to override that option
  will be stuck on the old setting.

* It moves us towards a point where we can get rid of the super-surprising
  feature of synapse where the default settings for the config come from the
  generated yaml.

* It makes setting up a test config for unit testing an order of magnitude
  easier (see forthcoming PR).

* It makes the generated config more consistent, and hopefully easier for users
  to understand.

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

Improved federation configuration docs.  Specifically detailing  .well-known and SRV based delegation methods. 

Inspiration Valentin Lab <valentin.lab@kalysto.org> for https://github.com/matrix-org/synapse/pull/4781

* Clarify what registration_shared_secret allows for (#2885)

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

* Add changelog

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

* 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

The v4v6 option only has a usage one ipv6 socket: https://serverfault.com/q/747895



Signed-off-by: Flakebi <flakebi@t-online.de>

Signed-off-by: Joseph Weston <joseph@weston.cloud>

So that it actually works. See https://httpd.apache.org/docs/2.4/mod/mod_proxy.html#proxypass



Signed-off-by: Paul Tötterman <paul.totterman@iki.fi>