feat(backups): implement long term retention

[fbeauchamp]

long term retention sometimes called GFS ( Grand Father / Father / Son) is a way to promote some backup to be kept on a long time

that way , the user can use the find the best equilibrium between storage and security

This commit add the code mechanics to indentify backup that can be deleted safely. It is intended to use with a form that ask the user for the number of day, week, month, and year for which XO will keep the most recent

It extends the actual system of keeping the n most recent backup

Keep in mind that the backup promoted by week and month can be decaled

Description

Short explanation of this PR (feel free to re-use commit message)

Checklist

• Commit
  • Title follows commit conventions
  • Reference the relevant issue (Fixes #007, See xoa-support#42, See https://...)
  • If bug fix, add Introduced by
• Changelog
  • If visible by XOA users, add changelog entry
  • Update "Packages to release" in CHANGELOG.unreleased.md
• PR
  • If UI changes, add screenshots
  • If not finished or not tested, open as Draft