get our sass files ready to be able to remove jammit

this makes every entry in assets_real.yml just a reference
to one file in app/stylesheets/bundles/<bundle_name>.scss

refs: CNVS-18715

STILL NEED TODO:
* tinyMCE (in next commit)

Test Plan:
- Nothing front-end facing changes
- CSS should compile successfully

Change-Id: I5d732df6b7be44996157612ad178f86a242c3550
Reviewed-on: https://gerrit.instructure.com/49153
Tested-by: Jenkins
Reviewed-by: Colleen Palmer <colleen@instructure.com>
Reviewed-by: Ryan Shaw <ryan@instructure.com>
Product-Review: Ryan Shaw <ryan@instructure.com>
QA-Review: Ryan Shaw <ryan@instructure.com>

Author: ryankshaw