Use new rights mixin

requires !1131 (merged)

• Migrate existing rights (duplicated rights)
• Update the fixture (duplicated rights)
• Re-implement get_corpus for the new mixin
• Update endpoints/tests to work with the new mixin

added 1 deleted label

changed the description

mentioned in issue #594 (closed)

mentioned in merge request !1139 (merged)

marked the checklist item Update endpoints/tests to work with the new mixin as completed

closed

mentioned in commit e603e5a7