Pass dimensions into admin_thumbnail
Merge remote-tracking branch 'upstream/next' into next
Add an empty block just before the fieldsets so one can easily add some controls here.
Implement the admin_urlname protocol on ContentModelMixin to make it easier to generate an admin_url to a base content.
fix medialibrary admin to work with new extension mechanism
Make stock VideoContent easier to extend by subclassing.
add documentation for extension mechanism and mediafile admin widgets
Merge branch 'master' into next

* master:
  add documentation for extension mechanism and mediafile admin widgets
  fix medialibrary admin to work with new extension mechanism