This was never used, so let's get rid of it.

Add ability for ASes to /login using the `uk.half-shot.msc2778.login.application_service` login `type`.

Co-authored-by: Patrick Cloke <clokep@users.noreply.github.com>

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.

This reverts commit 0122ef10.

This was incorrectly merged to master.

This reverts commit 319c41f5, reversing
changes made to 229eb814.

That fallback sets the redirect URL to itself (so it can process the login
token then return gracefully to the client). This would make it pointless to
ask the user for confirmation, since the URL the confirmation page would be
showing wouldn't be the client's.

Record how long an access token is valid for, and raise a soft-logout once it
expires.

It doesn't really belong under rest/client/v1 any more.

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