Cleanup any nulls on is_published_in_era columns on Draft tables and make non-nullable

[mbarnett]

For some reason we originally defined these columns as nullable, default false booleans, which has the weird side-effect of allowing nulls to creep into this column if they're written in explicitly, complicating queries that attempt to identify draft vs published records, because it has to account for this three-valued logic.

There's no real reason to allow this column to be nullable, so we need a simple migration to add a non-nullable constraint to the column

[mbarnett]

Closing, since I think this got merged.