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, we would want another table. Otherwise you would need to do content parsing in order to find out which links are to be displayed at any given position, which is much less efficient.

I'd use a multiple selection bo.