Merge pull request #4019 from matrix-org/dbkr/e2e_backups

changelog.d/4019.feature

synapse/api/errors.py

synapse/handlers/e2e_room_keys.py

synapse/rest/__init__.py

synapse/rest/client/v2_alpha/room_keys.py

synapse/server.py

synapse/storage/__init__.py

synapse/storage/e2e_room_keys.py

synapse/storage/schema/delta/51/e2e_room_keys.sql

tests/handlers/test_e2e_room_keys.py