UX/I Polish for Files Preview

fixes CNVS-15877

This adds ux polish to more closely match the comp of our new File
Previewer.

QA Test Plan:

- Load up file previewer
- Click on an file and then click on the magnifying glass to preview
- Things should now look cleaner, more usable, and look closer to the
ux design comp, with the exception of these design changes:
  - We use different arrow icons
  - The thumbnail preview bar stretches the full window width

Screenshot of design comp: http://cl.ly/image/2B460H0J3u30

Change-Id: I24e586041db6081e3c600e2ccbfc92e41cba2d8f
Reviewed-on: https://gerrit.instructure.com/42742
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Clay Diffrient <cdiffrient@instructure.com>
Product-Review: Clay Diffrient <cdiffrient@instructure.com>
QA-Review: Clay Diffrient <cdiffrient@instructure.com>