add outcome drawer icons

test plan:
go to stylguide (http://localhost:3000/styleguide#icons)
and verify that .icon-arrow-open-right and .icon-arrow-open-left
icons are visible and that classes are correct

Change-Id: Iac2eb10c179d86e28ea2c5d453d769a95ade7ccc
Reviewed-on: https://gerrit.instructure.com/29865
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Matt Fairbourn <mfairbourn@instructure.com>
Reviewed-by: Braden Anderson <banderson@instructure.com>
Product-Review: Patrick Cox <rpatrickcox@gmail.com>