Create idea of read-only database

What does this MR do?

Instead of checking if the node is a Geo secondary, check if the node is a read-only node. This idea will be backported to CE.

Why was this MR needed?

• Closes gitlab-org/gitlab-ee#2855
• Closes gitlab-org/gitlab-ee#3398
• Closes gitlab-org/gitlab-ee#3399
• Closes gitlab-org/gitlab-ee#3505
• gitlab-org/gitlab-ee#3402 (unrelated)
• gitlab-org/gitlab-ee#3444 (unrelated)

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

What are the relevant issue numbers?

Related to gitlab-org/gitlab-ce#37534 (needs CE MR to close it)