Configure saving drafts on/off per account: Bug #726728

Saving drafts may not be configured per-account.  This is to assist
users who (a) are using Gmail clients that include trashed messages
in their conversations (which is where Geary's drafts wind up when
discarded) or (b) Gmail users where, for unexplained reasons, Geary
can't expunge their draft messages (bug #729136).