Link to encoding confirmation view from file items

* Add confirm button to file items that are waiting for confirmation
* Preselect file in encoding confirmation view
improve encoding confirmation blank slate
improve confirm encoding view

* do not double check if adding a preselected file already triggered a
  check
* ensure selection and summary html are reset after save
* for the save callback to fire, the reponse has to be valid json not
  just a HEAD
Merge pull request #113 from tf/confirm-encoding-view

Improve Confirm Encoding View