Merge pull request #2352 from matrix-org/erikj/group_server_split

synapse/federation/transport/client.py

synapse/federation/transport/server.py

synapse/groups/__init__.py

synapse/groups/attestations.py

synapse/groups/groups_server.py

synapse/handlers/room_list.py

synapse/http/server.py

synapse/server.py

synapse/storage/__init__.py

synapse/storage/group_server.py

synapse/storage/schema/delta/43/group_server.sql