:construction: Integrate merchant gem

Mustache is just too painful because you can't see half of the errors.

The almost-empty view in app/views is needed for rspec to run at all (makes me a bit sad). With some logic in the view finding code we should be able to make sure that it never gets displayed to visitors.

This brings up the question: do we test client-specific views, and how do we do it in a maintenable question? Definitely something to keep in mind as we proceed.

I'm keeping Product inside the catalog for now until I add more classes and I figure out how I want to lay them out.

I am a bit worried about the amount of setup code for the catalog, though. Any ideas in that area would be greatly appreciated (how to rearrange the whole object graph).

## Todo

- [ ] Remove mustache view objects
Good point, but I'm not sure what a better alternative would be. I don't want to do it in the controller because I'm pretty sure it will be needed in multiple places (and I also kinda want to hide it a bit).