Purging Central resources via CLI

[matthew-white]

Currently, it's possible in Central to purge forms and submissions that have been soft-deleted. Purging is done via a command-line script that's automatically run once a day. By default, a form or submission is purged 30 days after it's deleted. However, if manually run with the correct flag, the command-line script can be used to purge deleted resources immediately (not waiting for the 30 days). It's also possible use the script to immediately purge only a subset of deleted resources (e.g., a single form).

This issue is to document this command-line script for users. The goal is to give server admins a way to immediately purge resources if that's needed. We talked about adding an endpoint for purging (see getodk/central-backend#739). However, our currently thinking is that we don't want to make immediate purging too easy. Yet we still want to make it possible for server admins, which is why we want to document this command-line script.