Disintegrate EntityRole and EntityLink

https://redmine.teklia.com/issues/5980

Remove the EntityRole model
Remove the EntityLink model
Remove ListCorpusRoles
Remove CreateEntityRole
Remove ListElementLinks
Remove CreateEntityLink
Remove parents and children from the entity serializer
Remove the entity_link and entity_role tables from corpus exports
Bump the export version
Remove support from the load_export and merge_entities commands