Drafts were saved without In-Reply-To and References, and a draft
re-opens in compose mode, where the composer has no original message to
rebuild them from. A reply that was saved and re-opened, or sent and
brought back with Undo, therefore went out as a new conversation.
`createDraft` now stores the threading headers, the composer saves them
with every draft of a reply, and re-opening a draft carries them into
the composer for later saves and the send.