Put back broadcast messages to sign-in page for self-hosted

What does this MR do and why?

Related to Broadcast Message no longer visible on login sc... (#419793 - closed)

Summary

Since GitLab 16.0, Broadcast Messages are no longer visible on the login screen, but they remain visible once signed in.

What is the expected correct behavior?

Broadcast message should be seen on the login screen.

For SaaS

• Broadcast messages should not display on /signup or /signin.

For Self-Managed

• Broadcast messages should be allowed to display for both /signup and /signin as each admin has the ability to configure these for their individual instance.

See full discussion and reasoning here.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The original MR has been deployed to GitLab.com (not applicable for documentation or spec changes).
• This MR has a severity label assigned (if applicable).
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the e2e:package-and-test-ee job has either succeeded or been approved by a Software Engineer in Test.

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