More responsive article layout, still quite a way to go. Refactored main.js with separate file for viewmodels. Started viewmodel for add link form as binding is more complex with image validation and stuff

Next tasks:
- Allow adding of image, if URL is image, or show alternate image URL if article URL is not an image
- make show comments responsive
- Make add form really validating
- JavaScript: dependency injection for data service
- JavaScript: better design for article & comments factory
- JavaScript: clean up articleviewmodel (do not extend, when overriding quite some properties anyway)