fix file preview for "only visible with link" files
fix file preview for "only visible with link" files

test plan:
 - as a teacher, upload an image file to a course's files page
   and set it "only visible with link"
 - as the teacher, click the file and copy the preview URL from
   the address bar (it should look like /courses/1/files?preview=1)
 - as a student in the course, paste that into the address bar.
   you should see the file, not a blank white page
 - there should be no left/right previous file/next file
   navigation buttons (since this file has no place in the
   file list, those don't make sense)

fixes ADMIN-1544

Change-Id: Ide117236202be85628659abbcf8744e8ac1e9cee
Reviewed-on: https://gerrit.instructure.com/168730
Reviewed-by: Carl Kibler <7c2b40129dd885a147d5b2dda56dda016b9a8f57@instructure.com>
QA-Review: Carl Kibler <7c2b40129dd885a147d5b2dda56dda016b9a8f57@instructure.com>
Tested-by: Jenkins
Product-Review: Jeremy Stanley <b3f594e10a9edcf5413cf1190121d45078c62290@instructure.com>

sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,djbender/canvas-lms,djbender/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,djbender/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms