Given that several attempts at the BS4.a6 upgrade have fallen flat
with fiddling around with django-compressor we shall pivot to building
via gulp.

Statics that require pre/post/whatever-processing will be sourced from
src/whatever and outputted to dist/post/whatever. Django's staticfiles
will then source static files from dist/post into the existing
dist/static directory—appending a cachebusting hash.

Removed traces of django-compressor. manage compress lines are now
gulp deploy and come **before** manage collectstatic.

Upgrade to 6.x branch via node maintained apt sources. This is done in
xsacdb-image v0.4.0-1. Bump to that.

We're using npm, ignore required in .gitignore.

We need a package.json file now. Noticed we have a gemfile, unsure if
that's still needed (surely we now use node's sass binary).

We also need npm install in test and build steps. Include in 'bower' job
(to avoid creating too many jobs) and rename that job to frontend.