for whitelabeled edition, show branding name (e.g. 'ownCloud') instead of appname in header bar

@MTRichards @karlitschek please check if that’s good. Works well on desktop and on mobile without having too much stuff in the header.

![enterprise-owncloud](https://cloud.githubusercontent.com/assets/925062/4421366/5115e122-4583-11e4-9dc7-d9cb65800fb8.png)