Revert "Bug 964862 - [Messages][Drafts][Refactoring] Move MessageManager.draft to ThreadUI.draft"

This reverts commit df7b1fe4d0d93d5e3b9621fface6b091eb7ea84e.
Bug 964862 - [Messages][Drafts][Refactoring] Move MessageManager.draft to ThreadUI.draft r=rwaldron