Video "Additional Files" list more pure

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