Respect mount point

This is intended as a fix for #287. I've fixed serving of all assets except the fonts and made sure all view URLs are generated using the `url` helper method.

What still needs to be done is
* [ ] find a way to serve the fonts correctly
* [ ] find a way to fix [this](https://github.com/Koronen/stringer/blob/878faed15213b7db507a5ba402ff91fd655ddd28/app/public/js/app.js#L322) url

Any suggestions are welcome!