Separate npm install from npm build

[znarf]

Right now, we have a postinstall npm hook which is automatically triggering a build after we run npm install. I think we should remove that and clearly separate these tasks.

My understanding is that we're doing that mainly for the Heroku setup. We'll need to have that in mind if we proceed to change something.

https://devcenter.heroku.com/articles/nodejs-support#heroku-specific-build-steps