A bunch of style/polish for newfiles

fixes: CNVS-15244 CNVS-15261 CNVS-15374

changes:
handles overflow by scrolling
matches blake's mockup

test plan:
try the folder tree in both new files and by pressing
the "add image" icon in the tinyMCE toolbar and going
to the "From Canvas" tab.

TODO: 'active' style

it should look good and work well.

Change-Id: I6219da771758fd00d6ed3005ed459097a83fcd7e
Reviewed-on: https://gerrit.instructure.com/40922
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Clay Diffrient <cdiffrient@instructure.com>
Product-Review: Ryan Shaw <ryan@instructure.com>
QA-Review: Ryan Shaw <ryan@instructure.com>