Merge pull request #3630 from matrix-org/neilj/mau_sign_in_log_in_limits

changelog.d/3630.feature

synapse/api/errors.py

synapse/app/homeserver.py

synapse/config/server.py

synapse/handlers/auth.py

synapse/handlers/register.py

synapse/storage/__init__.py

...se/storage/schema/delta/50/make_event_content_nullable.py

tests/handlers/test_auth.py

tests/handlers/test_register.py

tests/storage/test__init__.py