[Zotero RDF] Export/import encyc., dict., & conf. article containers

See https://forums.zotero.org/discussion/29970/faulty-rdf-mapping/

I think I made a more extensive list of what goes missing after export -> import, but I can't find it now.

One thing that I did notice is that Access Date get copied into Date if Date is missing. That's because we use dcterms:dateSubmitted for Access Date export, but allow dcterms:dateSubmitted as a fallback Date for compatibility with some websites. dcterms:dateSubmitted for Access Date is a hack and we should use something more appropriate. I couldn't find anything, so perhaps we could add it to Zotero namespace (or somewhere else).