Merge remote-tracking branch 'origin/develop' into rav/event_auth/4

changelog.d/6220.feature

changelog.d/6317.misc

changelog.d/6357.misc

changelog.d/6359.bugfix

changelog.d/6361.misc

changelog.d/6363.bugfix

docs/sample_config.yaml

snap/snapcraft.yaml

synapse/__init__.py

synapse/_scripts/register_new_matrix_user.py

synapse/api/errors.py

synapse/config/captcha.py

synapse/config/emailconfig.py

synapse/config/server.py

synapse/federation/federation_client.py

synapse/federation/transport/__init__.py

synapse/federation/transport/client.py

synapse/federation/transport/server.py

synapse/handlers/auth.py

synapse/handlers/directory.py