Implement static pages

Uses the new static page api now in public website (https://github.com/moneyadviceservice/public_website/pull/89).

``ArticleDecorator`` has been turned into a generic ``ContentItemDecorator`` as the requirements for decorating a static page were almost identical to that for an article.

Potentially we might be able to reduce some duplication with the entities as ``Core::StaticPage`` is almost identical to ``Core::Article``.