Implement New Files Preview

This is the first commit of preview functionality into New Files
it isn't polished, but should be a good starting point for
polish.  Tickets are being created to account for any remaining
tasks.

closes CNVS-14727

Test Plan: (sorry kinda big)
  - Enable "Better File Browsing" for a course
  - Go to course files (/courses/##/files)
  - Make sure you have some images uploaded... since that's all
    that is really supported.
  - Click on a file (but not on the file name itself)
  - It should highlight
  - Click the "View" option in the toolbar that should appear
  - An overlay should cover the screen
  - You should be able to tab to each of the interactive
    elements on this page
  - Clicking Download should download the file.
  - Clicking Info should open the information panel.
  - Clicking the show button at the bottom should open up the
    preview thumbnails
  - The arrow keys (and the buttons on the screeen) should
    navigate left/right
  - A non-image file should show you a nice message telling
    you it can't be previewed yet.
  - Clicking close should, you know... close the panel.
  - Now select multiple files and click "view" again.
  - Perform all the same tests, but now you should be limited
    to only the items that you selected.

Change-Id: I8f48a133f739b9bddda5e340f5156269691c7870
Reviewed-on: https://gerrit.instructure.com/41451
QA-Review: Jahnavi Yetukuri <jyetukuri@instructure.com>
Reviewed-by: Ryan Shaw <ryan@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Cosme Salazar <cosme@instructure.com>