Add support for text/pgp.

Summary:
If Someone sends an encrypted attachment without any content, we should
display the content of the encrypted attachment.

Test Plan:
show one text/pgp encrypted attachment correctly (included as testdata).

Also make sure, that a "fake" text/pgp attachment don't break the viewer.
This part is not working at the moment, I'll look at it later...

Reviewers: #kde_pim, vkrause

Subscribers: #kde_pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D8521