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.
Hmm, I'm not sure where it is unnecessarily I guess; the only times I can think of where it should be set are on import and when submitting a page.