Shows an alert if the user opens in preview mode a post with unsaved changes.

Fixes [this issue](https://github.com/wordpress-mobile/WordPress-iOS-Editor/issues/426).

Known issues:
- (Low Priority) [This](https://github.com/wordpress-mobile/WordPress-iOS-Editor/issues/429).
- (High Priority) [This](https://github.com/wordpress-mobile/WordPress-iOS-Editor/issues/430).  I'm going to be working on this one next.  We have seen similar issues already so I feel this problem is not really introduced by this PR.

/cc @bummytime