Integrate OpenLibrary.press <=> Lenny callback

[mekarpeles]

Now that https://github.com/ArchiveLabs/openlibrary.press has working purchase + file download capabilities, create an integrated purchase flow between Lenny and openlibrary.press, which allows a Lenny instance to pass along a callback url as a meta parameter to the stripe payment flow so that after the transaction succeeds, the openlibrary.press server can then can callback the Lenny endpoint to seamlessly add a new file.

[mekarpeles]

This should be done, we're just blocked on testing because openlibrary.press currently is blocked from POSTing out from its restrictive network.