Support encrypted mails forwarding

Summary:
Some notes:

• What we do is: if the mail is encrypted, decrypt it and copy its content into a new message (with plaintext, html and attachments, if any), and use this message as attachment for forwarding
• The isEncrypted function from KMime doesn't seem to detect every kind of encrypted mails. AFAIK this structure is not detected:
  • multipart/mixed
    • text/plain
    • application/pgp-encrypted (attachement, named "ATT00001")
    • application/octet-stream (attachment named "encrypted.asc")

Reviewers: cmollekopf

Tags: PHID-PROJ-6npnfcmppynqynn7slmv

Maniphest Tasks: T8112, T7024

Differential Revision: https://phabricator.kde.org/D10966