Merge branch 'master' into develop

CHANGES.md

MANIFEST.in

changelog.d/3995.bugfix

changelog.d/3996.bugfix

changelog.d/3997.bugfix

changelog.d/3999.bugfix

changelog.d/4008.misc

changelog.d/4017.misc

changelog.d/4019.feature

changelog.d/4022.misc

changelog.d/4027.bugfix

changelog.d/4033.bugfix

docker/conf/homeserver.yaml

synapse/__init__.py

synapse/config/emailconfig.py

synapse/push/mailer.py

synapse/res/templates/mail-Vector.css

synapse/res/templates/mail.css

synapse/res/templates/notif.html

synapse/res/templates/notif.txt