replace compass with node-sass

after many steps towards this moment, we're finally here

This yanks sass and compass out of canvas-lms
completely and instead uses the libsass based
node-sass to compile our SASS files.

wins:

It is WAYYY faster!
as in, < 10 seconds to recompile all css in canvas
(compared to the 5+ minutes it used to take)

It is all in JS, helping use move to a completely
nodeJS based fronted tooling workflow.

next steps:

remove jammit: we don't need an assets.yml file
since node-sass can output compressed css for us
and we use sass to do all of our @import'ing of other
files (@colleen calls those "compiler" sheets), this
would simplify and speed up fronted asset building
even more

use gulp/broccoli/whatev to do cached, incremental builds

test plan:

all outputted css should look exactly the
same as it used to.

run `npm run compile-sass`, make sure it works
and is way faster than `rake css:generate` used to be

Change-Id: I7d865ea6b3e374cdc27a883d2019a4c15746c0e2
Reviewed-on: https://gerrit.instructure.com/38416
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Ryan Shaw <ryan@instructure.com>