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.
No routes will be required. But again, if we check that in the entity, we could get false positives just by trying to visit the ACP page. That would be a validation check probably done in the ACP controller?