Static compilation

PR goes towards not re-compiling app on static asset changes. Mainly

 * On dev, include `head` css externally, not inline, and don't add a hash to the static filenames
 * Tidy up the filenames produced by the `hash` task, so we can use `sass --watch`, e.g.

```
sass -I common/app/assets/stylesheets/components/normalize-scss/ -I common/app/assets/stylesheets/components/sass-mq/  --watch common/app/assets/stylesheets:static/target/hashed/stylesheets
```