legacy-display: accept text/plain legacy display parts
https://www.ietf.org/id/draft-autocrypt-lamps-protected-headers-02.html
Makes it clear that the "Legacy Display" part of an encrypted message
with protected headers can (and indeed, should) be of content-type
text/plain, though some clients still generate the Legacy Display part
as content-type text/rfc822-headers. Notmuch should recognize the
part whichever of the two content-types it uses.
See also discussion in
https://github.com/autocrypt/protected-headers/issues/23 for why the
community of implementers is moving in the direction of text/plain.
Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>