Fix tests / dev environment

This MR fixes the development environment and unit tests, so the instructions given in HACKING.md are valid again. The development environment works as described again, and the tests pass.

There is a handful of changes:

• Synapse user registration no longer retries forever (rewrote script in python).
• Synapse no longer rate limits C/S-requests.
• Tests now interact with cactusbot via Synapse's C/S API to register a sitename.
• Pin more than one level of python dependencies using pip-tools (fixes url-encoded routing bug in tests).
• Pin docker image to minor version (python:3.9.0 becomes python:3.9).
• Removed test test_query_room_alias_servername_with_underscores. Since that code path needs a real room alias (we could refactor application code to make this testable again).

This MR doesn't really add any new functionality - only fixes existing stuff. It also does not touch app.py at all.

I think it would be nice to add even more tests, especially stuff around power level replication. It would also be nice to run these tests in CI (so they don't go forgotten again).

But this MR is really just to fix the stuff that's broken, before adding new things.