code cleanup in FilePreview in newfiles

this doesn't change anything but
the main thing here is splitting out our
css file into more modular chunks for
each react component renaming things from
"ember" to "react". from now on,
try not to add new stuff to react_files.scss,
make a new scss file with the same name as the 
react component you are making.

Change-Id: I013d17d385ed8dadae98b9f186e0dfae8614169c
Reviewed-on: https://gerrit.instructure.com/42538
Reviewed-by: Clay Diffrient <cdiffrient@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Ryan Shaw <ryan@instructure.com>
QA-Review: Ryan Shaw <ryan@instructure.com>