switch react_files to react-router

fixes: #CNVS-14388
this fixes some of the hacky routing stuff I was doing

Change-Id: I81c730c80f9100e41941fc2b3615cfef92b92fea
Reviewed-on: https://gerrit.instructure.com/38812
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>