Compile SCSS on Heroku

[georgebrock]

As a temporary measure I committed the compiled CSS. This is fragile and irritating. We should change things so that the SCSS is compiled as part of the collectstatic management command which Heroku helpfully runs for us on deploy.

[jaylett]

It's unclear to me at the moment whether the correct approach is to have our own buildpack that does both pip & gem dances, or if installing the gems in a custom collectstatic command is acceptable. The latter sounds easier but more fraught. OTOH custom buildpacks are in labs.