EZP-22308: store in db sortkey value (as in legacy) and return it while ...

related to : https://jira.ez.no/browse/EZP-22308

When creating a relation using the PAPI, it never store the sort_key_string value.
Then, if you try to search for contents in relation with another content, it never return an empty result.

With this correction, sort_key_string is initialize with the value of the related content and when searching for content, it returns the good content.