Create boundOneWay util function

Closes #2958
- Created `boundOneWay` util to extract common pattern of having a oneWay property that doesn't break its binding after being set
- Changed `EditorControllerMixin.willPublish`,  `PostSettingsMenuController.publishedAtValue` and `.slugValue` to use the new `boundOneWay` util
Merge pull request #2960 from novaugust/boundOneWay

Create boundOneWay util function