Resolve difference between submitForm(), submit(), and save() in EntityFormController

Updated: Comment #12

When EntityFormController was first written, it had both submit() and save() methods, and they were attached directly to each button.
Then when EntityFormController was moved to implement FormInterface, it gained a submitForm() method.

Proposal

We tried moving all submit and save logic to submitForm(), and remove save() and submit() but this did not work because preview and save often require common code to run before they work. For an example see CommentFormController.

The current approach is to remove submit() and refactor into save() and submitForm() where appropriate. This means that for entity confirm forms the actions have been changed to not call the save() submit handler and only call submitForm().