Remove entities from exports

https://redmine.teklia.com/issues/11004

Requires backend#1921 (closed)

The export database changes made in the backend should be reflected in this library.

Additionally, a new section in the README should document how to migrate export databases from version 10 to 11, using a .sql file committed to the repo that performs the necessary table rewrites, and recommending a backup before migrating the database. No other older versions should be supported yet.