Add in support for the gamified UI

This pull request adds in support for a UI that is supposed to represent a game. For ease of implementation, the game consists of computing a score deterministically using the existing carbon footprint components, and the percentage of confirmed trips, and then mapping the score into an image. The score -> image mapping is intended to be non-linear to provide an increased challenge as the game progresses.

We haven't determined the mapping from the components to the score yet, so the result screen currently displays the individual components. The actual score calculation and the image mapping will be implemented in a subsequent checkin.

The main component of this change is the new gamification client. In order to support the gamification client, the following high level changes needed to be made. We anticipate that these changes will be useful for the new and improved data visualization as well.
- Change the carbon footprint calculator to return the footprint with and without air
- Change the carbon footprint calculator to return the footprint for a range instead of a hard-coded "last week" value
- Significantly improved carbon footprint tests. I am now convinced that the basic calculations are indeed correct
- Generalization of the study specific result screen concept