Bug 964862 - [Messages][Drafts][Refactoring] Move MessageManager.draft to ThreadUI.draft
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
Bug 959696 - [Messages][Drafts][Refactoring] Move draft saving on visibility change to ThreadUI, and only store drafts if new one is saved r=rwaldron