Rework how email verification works

[sandhose]

Tasks

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1. Remove the concept of 'primary email' and unverified emails
2. Add a new separate 'email authentication' with codes
3. Hookup the react frontend to the new email authentications
4. Stop requiring a valid email during authorisation flows
5. Store registration flows in the database
6. Remove the old email add/verification screens
7. Check for email uniqueness on registration
8. Make sure running registration flows are attached to the browser with a signed cookie
9. Have a proper error message on duplicate emails and on wrong email verification code
10. Button to retry the email verification in the registration flow
11. Screen to choose a display name
12. Use the new user registrations in the upstream OAuth 2.0 registrations
13. Config options to control what's required
14. Check for username uniqueness and email uniqueness again when completing the registration

[cloudflare-workers-and-pages]

Deploying matrix-authentication-service-docs with    Cloudflare Pages

Latest commit:	24999ef
Status:	✅  Deploy successful!
Preview URL:	https://e80833c5.matrix-authentication-service-docs.pages.dev
Branch Preview URL:	https://quenting-optional-email.matrix-authentication-service-docs.pages.dev

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