We do it this way round so that only the "owner" can delete the access token (i.e. `/logout/all` by the "owner" also deletes that token, but `/logout/all` by the "target user" doesn't).

A future PR will add an API for creating such a token.

When the target user and authenticated entity are different the `Processed request` log line will be logged with a: `{@admin:server as @bob:server} ...`. I'm not convinced by that format (especially since it adds spaces in there, making it harder to use `cut -d ' '` to chop off the start of log lines). Suggestions welcome.

Add new method ratelimiter.can_requester_do_action and ensure that appservices are exempt from being ratelimited.

Co-authored-by: Patrick Cloke <clokep@users.noreply.github.com>
Co-authored-by: Erik Johnston <erik@matrix.org>

Add new method ratelimiter.can_requester_do_action and ensure that appservices are exempt from being ratelimited.

Co-authored-by: Patrick Cloke <clokep@users.noreply.github.com>
Co-authored-by: Erik Johnston <erik@matrix.org>

While working on https://github.com/matrix-org/synapse/issues/5665 I found myself digging into the `Ratelimiter` class and seeing that it was both:

* Rather undocumented, and
* causing a *lot* of config checks

This PR attempts to refactor and comment the `Ratelimiter` class, as well as encourage config file accesses to only be done at instantiation. 

Best to be reviewed commit-by-commit.

* 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

Have all unit tests import from our own subclass of trial's unittest TestCase; set up logging in ONE PLACE ONLY