Bug 928254 - [Flatfish][Gallery] support 2 column layout for tablet

https://bugzilla.mozilla.org/show_bug.cgi?id=928254

1. use [data-view="xxxview"] to switch different layout
2. separate the data-view styles into gallery_small.css and gallery_large.css
3. add preview view for tablet (separate two columns when landscape)
4. customize gallery_large.css to meet table's  UX requirement