WIP: Allow to set text shown when Pages are disabled

What does this MR do?

Add a new application setting similar to existing help text setting so administrators can customize the message seen by users when GitLab Pages are disabled.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

GitLab currently shows GitLab Pages are disabled. Ask your system's administrator to enable it. in Project settings when Pages are disabled. However there are legitimate cases where administrators do NOT want or cannot enable it:

• the GitLab Pages functionality is provided differently (e.g. still using GitLab-CI, but deploying to external web server)
• organization policy may forbid use of GitLab pages
• technical limitations in environment where GitLab is deployed

In this case the default message is not appropriate and may confuse users.

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
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?