Don't require developers to define resource controllers

Created by: gracewashere

All of the controller logic for displaying resources lives in DashboardController.

Before this PR, in order to set up the routes for a resource, developers had to create empty DashboardController subclasses that looked like this:

class CustomersController < DashboardController
end

Instead, this PR overrides Object.const_missing to automagically define missing ...Controller classes as subclasses of DashboardController. That way, developers only need to create DashboardController subclasses if they want to override the controller's actions.

This solution requires overriding Object.const_get, which I'm not super thrilled about. An alternative would be to always create the empty resource controllers through a rails generator, but I think it would be annoying for a developer to have a bunch of empty controller classes laying around.