Improve migrations using triggers

This improves our migration helpers code so triggers are only created if possible, and dropping them happens only if necessary.

See https://gitlab.com/gitlab-org/gitlab-ce/issues/36633 and the commits in this MR for more information.

General Checklist

• 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