use builtin Redirect component of react-router

...now that it exits
closes: CNVS-14706

react-router introduced a Redirect component in 0.5.2, use that instead of the custom one we made in files

test plan:
going to courses/x/files/folder should redirect to courses/x/files

Change-Id: I97f552833859c114279a5a3fa9b39806ea243d81
Reviewed-on: https://gerrit.instructure.com/38919
Reviewed-by: Jason Madsen <jmadsen@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Ryan Shaw <ryan@instructure.com>
QA-Review: Ryan Shaw <ryan@instructure.com>