Merge branch 'develop' into cross-signing_federation

INSTALL.md

changelog.d/6254.bugfix

changelog.d/6259.misc

changelog.d/6271.misc

changelog.d/6272.doc

changelog.d/6294.misc

changelog.d/6300.misc

changelog.d/6304.misc

changelog.d/6306.bugfix

changelog.d/6307.bugfix

contrib/experiments/test_messaging.py

docker/start.py

mypy.ini

scripts/move_remote_media_to_new_store.py

scripts/synapse_port_db

synapse/api/auth.py

synapse/app/__init__.py

synapse/app/appservice.py

synapse/app/client_reader.py

synapse/app/event_creator.py