Merge pull request #3633 from matrix-org/neilj/mau_tracker

changelog.d/3633.feature

synapse/api/auth.py

synapse/app/homeserver.py

synapse/config/server.py

synapse/handlers/auth.py

synapse/handlers/register.py

synapse/storage/__init__.py

synapse/storage/client_ips.py

synapse/storage/monthly_active_users.py

synapse/storage/prepare_database.py

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

tests/api/test_auth.py

tests/handlers/test_auth.py

tests/handlers/test_register.py

tests/storage/test_client_ips.py

tests/storage/test_monthly_active_users.py

tests/utils.py