Add ability to remove a previously added PGP key in WhisperBack

When a user add a PGP key to WhisperBack, if before sending the bug report that person finally prefer not sending a public key, then the field cannot be left empty or reset.

A workaround is to manually remove everything in between ——~~BEGIN PGP PUBLIC KEY BLOCK——~~ and ——~~END PGP PUBLIC KEY BLOCK——~~.

It’s a corner usecase, but maybe a reset button would do the trick.

Related issues

Is duplicate of #7797 (closed)

Original created by @mercedes508 on 9227 (Redmine)

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