[WIP] Serving static assets via asset-rack

closes #1405
- adds asset-rack to package
- moves all assets previously served via express.static into a rack
- handles url generation for statically served assets
- updates the asset-helper for hbs-templates to get the right url

Missing:
- [ ] allowing asset-hostname and CDN-configuration in config.js
- [ ] setting Cache-header and verify with tests (see  #1470)
- [ ] tests to verify functionality when in subdir
- [ ] Grunt-task to precompile assets
- [ ] add storage to upload uploaded Assets directly to the configured CDN
- [ ] more tests