Refactor #849

Updated by mhrivnak almost 9 years ago

The fields owner_type and owner_id are no longer used in the repo_content_units collection 
 * Remove them from the internal API calls 
 * Remove them from the mongoengine indexes 
 * Create a migration to remove them from the current repo_content_units table 
 ** The migration should validate that there are no entries in the database where (repo_id, unit_id) do not form a unique key. A manual update of the database will be required in that case. 

 These fields are not in our docs or examples, should not have been exposed by the API to begin with, and are not in known use today.