WIP Sponsored content

@kenlim @kelvin-chappell Hi, here's my simple starter for 10 ...

 - Someone tags a bit of content with _sponsored/sponsored_ tag to represent advertorial stuff, and and also tag for the individual _sponsor_.
 - We map that to some meta-data in our Content model.
 - We render that in to our view templates.

http://content.guardianapis.com/action-for-children/csa-ask-action-for-children?format=json&show-tags=type,keyword

TODO

 - We need a list of sponsors.
 - Think about the section fronts.
 - Find a place in our repository for sponsor logos etc.