Annual rotation of S3 backups

[jpmckinney]

Per the Google Doc #50:

https://ocdsdeploy.readthedocs.io/en/latest/use/kingfisher-process.html#data-retention-policy says that once we start on the current year, we go back to older years and only keep one per year (the first in that year)

This process should be manually run annually.

A management command can be written to interact with S3 and give the user the information needed to decide which backups to keep, following the retention policy. This can be a simple command-line interface.

See the notes below the horizontal rule in this comment (e.g. about different logic for Digiwhist): open-contracting/deploy#153 (comment)