Can't add an optional PGP key in WhisperBack

Originally created by @mercedes508 on #11033 (Redmine)

In Tails 2.0, when clicking on ‘optional PGP key’ in WhisperBack a pop-up window offers to paste either the key-ID or the public key itself.

After either copying the key itslef or its ID, clicking on ‘OK’ to close that pop-up and proceed further doesn’t do anything.

So users can’t add their keys using that feature.