Add submitted formatter setting for user view mode

The Drupal Core theme setting for "user picture" (node or comment) is unusual. It affects all entity types and view modes simultaneously without any possibility for variation. It uses a hard-coded view mode 'compact'. It only works if there is a hard-coded field named user_picture and such a field cannot be created through the GUI (this last rule applies to nodes, but oddly not to comments).

Now that we have a field formatter, we should revert to the standard Drupal way. Add a field formatter setting to control whether to display a user picture, and which view mode to use.