Build assets on the server instead of committing them to this repository

[supertassu]

We currently have our assets committed into Git. It has many downsides:

• If I'm using development mode (auto rebuild, no purgecss etc) I have to remember to build the production version before committing my changes
• It creates merge conflicts
• Theoretically someone could modify compiled assets and add malicious code and no-one would notice

When we update our deploying process to be something else than a cronjob running git pull (https://github.com/UTRS2/utrs/milestone/3) we probably should add a build step that builds assets and remove them from Git.