Validate encrypted submissions are PGP compliant in sd-app

[emkll]

We should consider adding validation of the PGP payload downloaded from the SecureDrop server in sd-app before sending to sd-gpg.

This will ensure the payload is valid, and that the sd-gpg vm has the private key to decrypt it.

(edited by @redshiftzero to remove reference to script that is gone after https://github.com/freedomofpress/securedrop-workstation/pull/194 was merged)