Improve autosave in the browser

[azaozz]

Several improvements that can make this quite better:

• Try to better detect when a restore may be needed.
• Make in-browser autosaves a "higher priority" than remote/server autosaves. The in-browser content is fresher. If both are available, prefer/emphasis the in-browser data.
• Add some subtle, always present UI for restoring a post from in-browser autosave. This should (probably) be only available before the users start typing.
• Consider adding the in-browser data to the revisions or display a preview/diff with the current data in some other way.