use svgs for file thumbnaill icons

closes: CNVS-16094

test plan:
open newfiles, the icons next to the filenames
should look like: http://cl.ly/image/3V0t3G2C172G

same ones should be used for the drag-n-drop
feedback.

in a future commit, clay will use the same ones
in his preview modal

Change-Id: Ife0d9bde9b6cb063acbd578af9e1e108617e9293
Reviewed-on: https://gerrit.instructure.com/42267
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Dan Minkevitch <dan@instructure.com>
Product-Review: Ryan Shaw <ryan@instructure.com>
QA-Review: Ryan Shaw <ryan@instructure.com>