Allow short link slugs to be moved from Test Mode to Live Mode, or released

Request: Let merchants move a short link created in Test Mode to a Live Mode product, or delete a short link so its slug can be used again.

Why: Test and live share the same slugs, and a slug can never be edited, moved or freed, even if its product is deleted. A link created while testing can't become the live link, so a merchant who has already shared it is stuck with it pointing at a test checkout.