Deprecate licensors in favor of copyright-holders. Addresses #126

Renames ``licensors`` to ``copyright_holders`` (aka ``copyrightHolders`` in webview). This also puts in some backwards compatibility, that maintains ``licensors`` in the response data.

Use the following SQL statement to update the database.
```
ALTER TABLE document RENAME COLUMN licensors TO copyright_holders;
```

Addresses https://github.com/Connexions/cnx-publishing/issues/64