More responsive article layout, still quite a way to go. Implemented image loading handler and alternate image URLs. Finally show images

Next tasks:
- make metadata responsive
- 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)
- JavaScript: Mixin of "votable"