Skip loading application settings from the database when migrations are pending

What does this MR do?

Skips loading application settings from the database if migrations haven't been run

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

Why was this MR needed?

Schema changes to the application_settings table can break rake db:migrate

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?

Followup to https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/11800#note_31168041

Closes gitlab-ee#2361