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 
Well, I really don't think there is.  I was very careful to search project wide for the string " remove];" and checked all occurrences of it.  I can see your concern and to be honest I thought it could be better to delay this change at first - but when I saw that there were only a handful of calls of this method, and triple checking my change I'm now pretty confident it's not going to be a problem.