v3 of postpone patches

[David Bremner]

This obsoletes, and hopefully doesn't break

     id:1478196548-19596-1-git-send-email-markwalters1009 at gmail.com

Compared to Mark's original patches I added the check for encryption
tags discussed in that thread, and split the draft stuff to it's own
file.

I have to admit I (or someone else) still need to line by line review
this, but I have tested it a bit more while working on the encryption
tag check. On the other hand, splitting the draft functions out into
notmuch-draft.el did involve a certain amount of blind search and
replace.