Make date format configurable

It should not be "short" by default. When talking about dates, the way to get short is to remove the time. Drupal revisions inherently *need* time because they are so chatty.

Just updated the MR to cover 2 new places where the hard coded format was used and also added a field diff plugin for date fields.

I've rebased the MR to cover the latest changes in the dev branch.

Updating to use latest branch

Lots of work needed here, we'll also need tests and an update hook to set the new config.