Tails OpenPGP Applet can't clearsign text including non-ASCII characters

When using gedit to type text including non-ASCII characters, and then using Tails OpenPGP Applet to clearsign it, the resulting text doesn’t match with the original one, and thus the signature is bad.

Feature Branch: bugfix/7968-cant-clearsign-non-ascii-text

Related issues

Blocks #6507 (closed)

Original created by @mercedes508 on 7968 (Redmine)

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