Passing the model instance means that logic such as rendering thumbnails and generating URLs to edit pages ends up on the template. This is bad because we're going to want to populate these widgets through mechanisms other than rendering the template (e.g. setState on the widget's Javascript API), and that would end up duplicating the logic on the template.
* Avoid calling versioned_static from static form media definitions
Fixes#5632
Defining media definitions with `class Media:` means that those lines are evaluated on application startup; this means that they cannot contain calls to the Django static file finder, because ManifestStaticFilesStorage doesn't work until collectstatic has completed. Fix this by making those definitions into `def media(self):` methods.
* Add test to confirm that inheritance still works with dynamic form media definitions
Matt Westcott