Revert "Drop support for delegating email validation (#13192)" (#13406)

changelog.d/13406.misc

docs/upgrade.md

docs/usage/configuration/config_documentation.md

synapse/app/homeserver.py

synapse/config/emailconfig.py

synapse/config/registration.py

synapse/handlers/identity.py

synapse/handlers/ui_auth/checkers.py

synapse/rest/client/account.py

synapse/rest/client/register.py

synapse/rest/synapse/client/password_reset.py

tests/rest/client/test_register.py