Add full text to index for Zotero collections

Zotero is already able to extract full texts from attachments, so rather than ignore them (as we do now), we could 

- use [the relevant API](https://pyzotero.readthedocs.io/en/latest/#zotero.Zotero.fulltext_item) to cache the texts where available
- pull together the text from all attachments under a given record, and add them to the content for indexing