Add user callout for instances with registration enabled by default

What does this MR do?

See #237905

Adds an alert to warn admins that anybody can register on their GitLab instance. The alert is only shown when the logged in user is an admin and the "Sign-up enabled" (will be renamed to "Registration enabled" in future MR) setting is checked.

Once closed a record is added to the user_callouts table with a feature_name of 25. If you delete this record the alert will show again.

Screenshots (strongly suggested)

Desktop

Mobile

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