allow plugins to prepend routes

prepended routes go in config/pre_routes.rb

Change-Id: Ib064c59a7aeae1d71c7b021b963504ad57357d45
Reviewed-on: https://gerrit.instructure.com/39266
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Cameron Matheson <cameron@instructure.com>
QA-Review: Cameron Matheson <cameron@instructure.com>