Editor restoration fixes.

Fixes [this issue](https://github.com/wordpress-mobile/WordPress-iOS-Editor/issues/355).

Also restores the edit mode now.  We could eventually decide to restore the caret position too.

Opened a [new issue](https://github.com/wordpress-mobile/WordPress-iOS-Editor/issues/383) to apply the same fixes to our legacy editor.

Opened [another issue](https://github.com/wordpress-mobile/WordPress-iOS-Editor/issues/384) since restoring the editor seems to show some small animation jumps.  Trying to break the problems up into several issues to avoid huge PRs.

How to test the old issues and the new fix:

- Launch the app normally.
- Go to the editor, and open an existing post.
- Optional step: enter edit mode.
- With the post open, double tap home and kill the app from there.
- Relaunch the app.

Old behaviour: :banana: 
New behaviour: :+1: 

/cc @bummytime 
![giphy](https://cloud.githubusercontent.com/assets/154014/5035446/e98f09b8-6b3c-11e4-96b1-a43acf5b94b6.gif)