feat: add option to disable user signup on login flow page

[matthiastz]

Related Issue or Design Document

Checklist

• [x] I have read the contributing guidelines and signed the CLA.
• [ ] I have referenced an issue containing the design document if my change introduces a new feature.
• [x] I have read the security policy.
• [x] I confirm that this pull request does not address a security vulnerability. If this pull request addresses a security vulnerability, I confirm that I got green light (please contact security@ory.sh) from the maintainers to push the changes.
• [x] I have added tests that prove my fix is effective or that my feature works.
• [ ] I have added necessary documentation within the code base (if appropriate).

Further comments

See also: https://github.com/ory/elements/pull/108

[CLAassistant]

All committers have signed the CLA.

[aeneasr]

Great stuff, @Benehiko can you PTAL?