don't render a white blank page for unauthorized files users

closes: CNVS-15821

test plan:
* go to courses/x/files as a non logged in user
* it should show login form

* go to courses/x/files as a non logged in user in
  a course that that is public.
* it should show files

* go to courses/x/files as a student in a course
  that has the "files" tab disabled by the teacher.
* it should redirect to courses/x

* go to courses/x/files/folder/a/real/folder in a course
  that doesn't have the better_file_browsing feature
  flag enabled
* it should render a 404 "not found" page

* go to /files as a not logged in user
* it should show login form

Change-Id: If1700e3814c2f7b1f97daecf89577b020eda765a
Reviewed-on: https://gerrit.instructure.com/42251
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Dan Minkevitch <dan@instructure.com>
QA-Review: Jahnavi Yetukuri <jyetukuri@instructure.com>
Product-Review: Ryan Shaw <ryan@instructure.com>