Allow HS to send emails when adding an email to the HS (#6042)

changelog.d/6042.feature

docs/sample_config.yaml

synapse/config/emailconfig.py

synapse/handlers/identity.py

synapse/push/mailer.py

synapse/res/templates/add_threepid.html

synapse/res/templates/add_threepid.txt

synapse/res/templates/add_threepid_failure.html

synapse/res/templates/add_threepid_success.html

synapse/rest/client/v2_alpha/account.py

synapse/rest/client/v2_alpha/register.py

synapse/storage/registration.py