add search to files and convert to react

closes: CNVS-12642
closes: CNVS-11221

test plan:

nothing should have changed for courses/accounts that
have the feature flag 'better_file_browsing' turned off
but I guess make sure.

with it turned on, you should be able to browse the
files in your course, search for things, and re-sort.

it should paginate, download files you click on,
show thumbnails of files (where it has one) and you
and see the files usage in the bottom left.

things that I know dont work:
* the folder tree thing on the left is still
  just static html, not the actual files in your course.
* it does full page reloads when you go from
  looking at one folder to another or when you
  search for something. I am going to fix that.

I know there is still work to be done here to consider
the entire files section complete, but this is a solid
foundation so that sterling and I can collaborate and
work forward.

Change-Id: Ia79ddba4318b7d6e52b38a87e91f615552da250b
Reviewed-on: https://gerrit.instructure.com/37491
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Sterling Cobb <sterling@instructure.com>
Product-Review: Ryan Shaw <ryan@instructure.com>
QA-Review: Ryan Shaw <ryan@instructure.com>