Block level sharing on images/videos in articles

Block level sharing has finally came to videos and images in articles.

Number of changes:
1. Changed the ordering of the BodyCleaner
2. Separated all of the block level sharing css in to its own file
3. New caption styles for images and videos
4. Changes to BlockLevelSharing.scala.html to allow video embeds to link to the video's own page as opposed to a block on a page

# Videos
![screen shot 2014-11-13 at 11 51 25](https://cloud.githubusercontent.com/assets/2236852/5027837/6fa025bc-6b2b-11e4-8966-d6384edf3499.png)

# Pictures
![screen shot 2014-11-13 at 11 14 51](https://cloud.githubusercontent.com/assets/2236852/5027427/a984906a-6b26-11e4-90d0-3e256a4f50a5.png)
![screen shot 2014-11-13 at 11 15 18](https://cloud.githubusercontent.com/assets/2236852/5027429/aec7878a-6b26-11e4-9b98-cde2649be13f.png)

# New captions on supporting (no sharing options)
![screen shot 2014-11-13 at 11 18 38](https://cloud.githubusercontent.com/assets/2236852/5027439/d828edb2-6b26-11e4-82f8-76f570302e93.png)


We have helper classes for this already just apply `mobile-only` class to the element instead: https://github.com/guardian/frontend/blob/master/static/src/stylesheets/base/_helpers.scss#L38