Part of #9744

Removes all redundant `# -*- coding: utf-8 -*-` lines from files, as python 3 automatically reads source code as utf-8 now.

`Signed-off-by: Jonathan de Jong <jonathan@automatia.nl&gt;`>

 - Update black version to the latest
 - Run black auto formatting over the codebase
    - Run autoformatting according to [`docs/code_style.md
`](https://github.com/matrix-org/synapse/blob/80d6dc9783aa80886a133756028984dbf8920168/docs/code_style.md)
 - Update `code_style.md` docs around installing black to use the correct version

This is an attempt to fix #8403.

Calls `self.get_success` on all deferred methods instead of abusing `self.pump()`. This has the benefit of working with coroutines, as well as checking that method execution completed successfully.

There are also a few small cleanups that I made in the process.

These are surprisingly expensive, and we only really need to do them at startup.

* Break down monthly active users by appservice_id and emit via prometheus.

Co-authored-by: Brendan Abolivier <babolivier@matrix.org>

The 'token' param is no longer used anywhere except the tests, so let's kill
that off too.

Allow for the creation of a support user.

A support user can access the server, join rooms, interact with other users, but does not appear in the user directory nor does it contribute to monthly active user limits.

When we register a new user from SAML2 data, initialise their displayname
correctly.

This reverts commit ec716a35.