Add entity class and interface

I can remove tests from the PR if they make it too big, but I think they may help show the entity methods in action.

The real focus of scrutiny on this PR really should be in the page class and the migration for the table setup.

1. Outstanding issue to figure out: How to enforce unique "route" names.
So I think the logic we need here is:
if new page (no page id) or eisting page and not importing (current value !== '') and current value != new value
select 1 from pages where route = new value and id <> current page id
if there is any result, error