Drafts 2.0 — Draft system revisited

Drafts now are placed directly into the HTML response. The response contains the draft information as well as the original content saved in the database for the content being modified. The draft itself is placed into the redactor box rather than the current content.

This patch also reduces some of the [Draft Saved] notices where there is no content or the content has not yet been modified. And also improves the behavior of the [Reset] and [Delete Draft] buttons by replacing the content with the last-saved content rather than empty content.

This is one of the main blockers for #830, because the ::set() method of Redactor is inconsistent with the initial setup routine and does not handle entire HTML documents well. This new draft system allows for draft recovery without the Redactor ::set() method.