Use a moved_entities table to record external entities moved to the system source
Instead of using a negative eid in the entities table, move the record
to a new table so we don't have an interval with a missing eid in
entities.
Related to #4846892
../../doc/book usr/share/doc/cubicweb-documentation
../../doc/html usr/share/doc/cubicweb-documentation
../../debian/cubicweb-doc usr/share/doc-base/cubicweb-doc