Adds a library and API for retrieving and settings core application
settings.
Adds base Page and Page Template model, which can be used as a consistent
way of build models for front-end pages.
Updates the missing routes handler to use the new Pages model.
Adds views for handling all error types and upadtes the logic that
handles the errors.
Adds additional methods to the App API for setting and retrieving the
views to use when handling various errors.