Feature/4474 fix lightbox thumbnails

I fixed the lightbox. ;)

It now has:
- Only one row of thumbnails
- Thumbnails can be scrolled using arrows at the sides of the screen
- Thumbnails automatically scroll at a position, where the selected image will be centered, if possible
- also: fixed duplicate first image; the "largePhoto" was also shown in thumnails, causing lightbox to display it twice
Sadly nothing for the content view yet. Try looking around in https://github.com/diaspora/diaspora/tree/develop/spec/javascripts/app/views a bit (Run the tests with `rake jasmine`). Basically you need to setup a (fake) model and a content view for it, then assert that this function doesn't return the first item from the photos attribute of the model.