Add Fix for T14524 Kontact creates broken pgp signed mails

Wenn we construct a signed message, we want to make sure, that the mail
itself is parsable for other MUAs, so we fix CE7Bit messages that contain
quoted-printable-like character when signing (1efea1c00df92e43d0ff66073851f48471d9d76b)
But this behaviour we only should do if it is text we sign and not a
mimestructure (multipart/mixed) already.

Resolves: bifrost.kolabsystems.com/T14524
Maniphest Tasks: https://git.kolab.org/T1139