fix: add proceedings series item to Item Tree issues#1556

As a side effect fixes the content type mixup:

There might be an issue with proceedings-related content types in the REST API. Compare:

• http://api.crossref.org/works/10.1145/243199 - XML says <doi type="conference_title">, and uses proceedings_metadata tag, in the REST API we have type: proceedings
• http://api.crossref.org/works/10.19146/pibic-2017 - XML says <doi type="conference_title">, and uses proceedings_series_metadata tag, in the REST API we have type: proceedings-series
• http://doi.crossref.org/search/doi?pid=support@crossref.org&format=unixsd&doi=10.18662/lumproc - XML says <doi type="conference_series"> and uses series_metadata tag, the REST API does not ingest this at all.

According to @pfeeney, the only difference between proceedings_metadata and proceedings_series_metadata in the XML is that the latter can contain the parent container metadata (conference series), otherwise the content types are the same. So I suspect this is a bug in the REST API, and it should be:

• http://api.crossref.org/works/10.1145/243199 - type: proceedings
• http://api.crossref.org/works/10.19146/pibic-2017 - type: proceedings
• http://doi.crossref.org/search/doi?pid=support@crossref.org&format=unixsd&doi=10.18662/lumproc - type: proceedings-series