use sass to remove repetition in flexboxgrid.scss

this just rewrites flexboxgrid.scss to take
advantage of sass's features. it outputs exactly
the same css, just with less lines of source.

Change-Id: Ie201c3bd762c294969e7b25a230cdf23cce7a813
Reviewed-on: https://gerrit.instructure.com/40623
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Chris Hart <chart@instructure.com>
Product-Review: Ryan Shaw <ryan@instructure.com>
QA-Review: Ryan Shaw <ryan@instructure.com>