Save draft state in message header for now

Apparently, storing the draft state header in the encrypted payload
introduces difficulties in Enigmail due to restrictions from
Thunderbird. To stay compatible, this commit keeps the draft state in
the outer header for now, but we should move it into the encrypted
payload if the issue with Thunderbird is somehow resolved.

see: https://github.com/autocrypt/autocrypt/pull/376#issuecomment-384293480