Add read-only banner to all pages

What does this MR do?

When the database is in a read-only state, display a banner on each page informing the user they cannot write to that GitLab instance.

Closes gitlab-org/gitlab-ce#43937.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Closes gitlab-org/gitlab-ce#43937.