Support "mass subscription" by email functionality

[decentral1se]

Adapt https://git.autonomic.zone/ruangrupa/toolshed#add-users-keycloak-api-py (this code does work 🥳) to support a web form which takes one email address per email and signs them all up with UPDATE_PASSWORD/PROFILE and VERIFY_EMAIL required actions. Like mailman3 management interface when you need to do a mass sign up moment for on-boarding an organisation.