Rename "field_notes" to "notes" and move to baseFieldDefinitions

Do we want to name them comments to be more Drupal-ish?

A quick search suggests that Notes is a pretty standard name for this in other CRMs. I think Notes is better in part because it isn't a Drupalism. We should be optimizing the UX for a general user, not a Drupaler. While we could have the machine name be "comments" and the label be "Notes", I think that introduces more confusion than it solves for.

I think I want to make comment an optional dependency.