Video "Additional Files" list more pure

* Change of ContentGroup.display_style default from 'button' to 'list'
  everywhere, including on the model.
* Schema migration to disallow display_style=NULL and set a default of
  'list'.
* Video display page "Additional Files" list made to use
  ContentGroup.display_style button to determine how the items should be
  rendered, but:
* Data migration to set all items' .display_style to 'list'
  (TODO: UI to enable button-mode display of items to come in a seperate
  branch)