Repository cache

Our categories controller calls our ``CategoryReader`` interactor each request so we can't use standard view caching.

I'm proposing this as a solution: a simple repository wrapper using whichever caching mechanism Rails is configured with.

Another option would be to use some form of caching library for Faraday but this is a very simple approach and can work regardless of the underlying workings of the repositories.

@andrewgarner @pvcarrera