Indirect post title in editor

Closes #3179
- Add `titleScratch` property to `PostModel`.
- Changed references to `title` to `titleScratch`
I don't remember if I identified a case where the observer would fire and `title` would not be a changed attribute or if it was just defensive programming.  Either way, if `title` hasn't been set by the time `titleObserver` fires then I think it sounds reasonable to remove the check :+1: