Fixup bodyformat after decrypt / verify

* src/mail.cpp (Mail::decryptVerify_o): Update body format.
(set_body): New. Helper for all the format fallbacks with
body format.
(updateBody_o): Use set_body.

This fixes that all crypto mails are shown as HTML when
HTML is enabled because we use HTML for our placeholder.

• GnuPG-Bug-Id: T4639