Implement JSON serialization on Challenges

Install 'active_model_serializers' and implement on Challenges
controller.
Add Backbone.js and associated libraries
Initial implementation of Backbone cards
Install database_cleaner for testing

Install database_cleaner and configure for JS testing with
PhantomJS.
Implement routes for cards, tweak factories
Refactor Backbone to work with collections

Change settings on active_model_serializers to remove root.
Backbone now fetches from a nested cards resource, using
a collection.
Root: false on card serializer only
Fix nondeterminism in Capybara JS tests
Fix nondeterminism bug in test
Implement 'User sees series of cards'

Implement basic card advancing on the Backbone collection level.
Switch from challenge to card view/template rendering.
[Finishes #52304189]
Merge branch 'user_sees_series_of_cards'