Merge branch 'develop' into matthew/gin_work_mem

.gitignore

docs/admin_api/media_admin_api.md

docs/admin_api/purge_history_api.rst

synapse/events/snapshot.py

synapse/handlers/directory.py

synapse/handlers/federation.py

synapse/handlers/message.py

synapse/handlers/room.py

synapse/handlers/room_member.py

synapse/http/servlet.py

synapse/metrics/metric.py

synapse/replication/slave/storage/events.py

synapse/rest/client/v1/admin.py

synapse/rest/client/v1/room.py

synapse/rest/media/v1/media_repository.py

synapse/rest/media/v1/media_storage.py

synapse/rest/media/v1/preview_url_resource.py

synapse/rest/media/v1/thumbnail_resource.py

synapse/server.py

synapse/server.pyi