in newfiles: when you click an filename, preview it, don't download

fixes: CNVS-16921
closes: CNVS-16860

expected behavior:
* clicking a file name should open a preview of it.
  while previewing, you should be able to use the
  arrows to flip through other things in this folder.
  This means that clicking a filename selects that
  row and unselects all other rows.
* the "href" of the filename link should still be
  the download url. this means that you can right
  click and "open in new tab" or "save link as" to
  download the file.
* if you have only selected folders, and you click the
  "preview" eye. it should open the preview and show
  all the "files" (not folders) in the current folder.

also test:
* clicking the "eye" preview button in the toolbar
  should work just like it used to.

Change-Id: I468359905ede6d32f750c3896c3eb364eb0b5c3c
Reviewed-on: https://gerrit.instructure.com/44483
Reviewed-by: Clay Diffrient <cdiffrient@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Jahnavi Yetukuri <jyetukuri@instructure.com>
Product-Review: Ryan Shaw <ryan@instructure.com>