Refactor page model into separate Feature and About page models

We are subclassing the controller and making the view path for both About and Feature be plain /spotlight/pages so we can reuse them across the controllers.

We now have a Page concern that is mixed into both of the FeaturePage and AboutPage models and they share most of their database migration as well.

This is a prerequisite for #14 and #27 and should unblock both of those tickets.