Add setting in the UI for governing admin approval for new signups

What does this MR do?

For #257881 (closed)

This adds a setting in the admin UI for controlling the behaviour of new user registrations. This setting is disabled by default.

• if enabled, admins will be required to manually approve any user that signs up on the instance, before they can start accessing GitLab.
• if disabled, no admin approval is required (this is the current behaviour)

This is behind a feature flag since we are developing this feature across multiple MRs. The flag will be removed once the whole feature is ready.

Changelog is skipped since this is behind a feature flag, hence there are no visible changes to customers.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team