Prevent creating multiple ApplicationSetting by forcing it to always have id=1

What does this MR do?

This approach prevents writing duplicates. We force that every new object created to have id set to 1 which is primary key for the table.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • 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?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/34728

cc @stanhu @nick.thomas @yorickpeterse