Improve Data Model of Sponsored Tags

Gives an admin view of sponsored tags ordered by tag rather than by line-item.
This also shows where tags are being targeted that don't actually exist on the site.

![image](https://cloud.githubusercontent.com/assets/1722550/5473926/1c6617ae-8605-11e4-84c4-b0a9b1475b97.png)


Next step is to totally replace `Sponsorship` data model with new `PaidForTag` model.