Feature/student loans comparison module

Added a comparison module.

An array in the `comparison` property of the json will load multiple collections, with each entry appended to the `filter_by` param for each request to create to comparable datasets and render them onto the same graph.

Example at `/performance/student-finance/comparison-of-applications-by-stage`

https://www.pivotaltracker.com/s/projects/911874/stories/67903726