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 
@diegoreymendez I know we talked offline about this...pushing the responsibility of saving the context to the caller. Is there an chance of introducing bugs elsewhere in the app with this change (outside of the editor).?