[BE] Create backfill migration for code hotspots, MVP release

Discovery Issue: https://gitlab.com/gitlab-org/gitlab-ee/issues/12683

We need to create and plan a backfill migration before the code hotspots MVC release:

• Collect all commits in the last 30 days, with latest commits first and create a analytics file edits record for each, along with a file record if needed
• Determine performance of script:
• how much time the whole back-filling will take,
• whether and which follower db to use
• impact on running application?
• impact on EE?
• write a scheduler migration