partially addresses remaining backend work for cmu-delphi/covidcast-indicators#1452

Prerequisites:

• Unless it is a documentation hotfix it should be merged against the dev branch
• Branch is up-to-date with the branch to be merged with, i.e. dev
• Build is successful
• Code is cleaned up and formatted

Summary

Add a function to database.py which, given a data source of covidcast rows to be deleted will:

• figure out what covidcast id belongs to each row
• figure out whether the row to be deleted is flagged for is_latest_issue
• delete the corresponding row from covidcast
• set the is_latest_issue flag on the next-most-latest issue for any latest entries deleted

Includes tests using both CSVs and tuples as inputs; for CSVs you need to have FILE privileges granted to the database user. I have the Dockerfile config change stashed in another branch for this, but I'm not sure whether we want to apply it in prod, so I'm having CI skip the associated test for now.