Matrix Chatbot (#13355)

* Add first version of the Matrix bot

* It's a stupid but necessary change…

* Dont list it twice

* All hail the linter!

* More linter-pleasing

* Use the correct user ID

* Add expression commands

* Add tests for new validators

* Fix room alias handling

* Wording

* Defer setup

* Simplify commands

* Handle exceptions

* Update requirements

* Review

* Move login back to constructor

* Fix review comments