Add ability to immediately purge a deleted form

[matthew-white]

When a form is deleted, it remains in the trash for 30 days, then is purged. It would be nice to be able to immediately purge a form that is in the trash. How about we add an endpoint for this?

One question I have is whether the endpoint should complete the purging itself or whether it should queue the purging to be completed as soon as possible by a worker. Maybe it depends on how long purging takes? nginx will time out a request after 2 minutes, so if purging a form can take longer than 2 minutes, I think we should make the purging happen asynchronously (to be completed by a worker). If purging always takes less than 2 minutes, then I think the endpoint could do the purging itself.

[matthew-white]

Closing this after discussion with @lognaturel. The current thinking is that we should document the CLI script for purging, but that we don't otherwise want to make immediate purging too easy. See getodk/docs#1868 about documenting the CLI script.