Merge lp://qastaging/~jamesh/unity-scope-mediascanner/video-aggregator-design into lp://qastaging/unity-scope-mediascanner

Implement new design of video aggregator scope.

Implement the video aggregator portions of the video aggregators design spec:

https://docs.google.com/a/canonical.com/document/d/1Y9H3SY2uwbeLZSta9bGdLcyGr-tP0LwHyIUigTnieww

There are a few areas where this is incomplete:

 * The category headings are supposed to have a clickable button to drill down to the corresponding scope. There is no support in the shell for this yet.

 * The design calls for result counts in searches. Again, we don't have a way for the subscope to provide this info. Even if we waited until all results from the subscope had been received, we've got no way to know whether the scope could have provided more.

 * It doesn't currently show any subtitle for results from subscopes. We'll need to have some agreement about how they structure their results for this to work.

It is still only querying the local videos and online videos scopes, but adding more is now trivial.

To get surfacing results, I'm passing the "featured" department ID in the subsearch. Aggregating the main surfacing results for the subscope would limit the designs it could provide, so this gives it a way to differentiate between the two cases. If there are better ideas for how to handle this, I'd be open to change it.