Send out emails with links to extend an account's validity period

changelog.d/5047.feature

docs/sample_config.yaml

synapse/api/auth.py

synapse/config/emailconfig.py

synapse/config/registration.py

synapse/handlers/account_validity.py

synapse/push/mailer.py

synapse/push/pusher.py

synapse/res/templates/mail-expiry.css

synapse/res/templates/notice_expiry.html

synapse/res/templates/notice_expiry.txt

synapse/rest/__init__.py

synapse/rest/client/v2_alpha/account_validity.py

synapse/server.py

synapse/storage/registration.py

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

tests/rest/client/v2_alpha/test_register.py