Allow hard deletion of subscribers through the API

[henrieger]

Using the API endpoint DELETE /subscribers/{id_or_email} just unsubscribes the subscriber, but does not delete it, which I find unintuitive. I was doing some tests where I was trying to hard delete a subscriber through this endpoint and wasn't able to, which bugged me. My feature suggestion is to be possible to delete a subscriber through the API, such that it actually doesn't exist in the dashboard afterwards, and for a separate endpoint to be created for unsubscription of subscribers, for example PATCH /subscribers/{id_or_email}/unsubscribe.

[jmduke]

This is a pretty reasonable bit of feedback. Two notes:

Unblocking yourself

You can create a bulk action with a type of delete_subscribers and that should do the trick (though I don't pretend that deleting subscribers through a completely different endpoint isn't janky!)

Actually fixing this

It'll be a breaking API change (and we'll need to cut a new version of the API for it), but now that we've expanded PATCH /v1/subscribers to handle subscriber type transitions it does, I think, make sense to make the DELETE endpoint actually delete. @catdevnull, mind throwing this on the backburner?