Improves closing and saving posts in the new editor

Fixes [this bug](https://github.com/wordpress-mobile/WordPress-iOS-Editor/issues/385).
Fixes [this bug](https://github.com/wordpress-mobile/WordPress-iOS-Editor/issues/388).

Also cleans up the code considerably.

/cc @bummytime 
Note to self: fix this... after a call to remove `self.post.original` shouldn't be used.