Refactor Pages into FeaturePages and AboutPages.

Refactor PagesController into FeaturePagesController and AboutPagesController which
subclass a common PagesController since the controller logic is essentially identical.

Move Page model logic into a Page concern which is mixed into the new FeaturePage and AboutPage models.
Merge pull request #77 from sul-dlss/page-model-refactor

Refactor page model into separate Feature and About page models