New feature flags

This page is a capture in the next bucket of the product backlog — a pre-sprint idea, not yet pulled into a sprint as a story.

• allow sign-ups. If true, users can create their own accounts. Done.
• auto-authorise sign-ups. If true, the account is automatically created. If false, admin user has to authorise it.

Notes:

• we need a UI to manage the UI options.