Merge pull request #5209 from matrix-org/erikj/reactions_base

changelog.d/5209.feature

synapse/api/constants.py

synapse/config/server.py

synapse/events/utils.py

synapse/replication/slave/storage/events.py

synapse/replication/tcp/streams/_base.py

synapse/replication/tcp/streams/events.py

synapse/rest/__init__.py

synapse/rest/client/v2_alpha/relations.py

synapse/storage/__init__.py

synapse/storage/events.py

synapse/storage/relations.py

synapse/storage/schema/delta/54/relations.sql

tests/rest/client/v2_alpha/test_relations.py