Server-side PDF indexing

[dstillman]

https://forums.zotero.org/discussion/comment/355785/#Comment_355785
https://forums.zotero.org/discussion/85650/how-to-index-pdf-text-using-only-zotero-online

Currently, if you add a PDF via the API or copy an attachment across libraries, it doesn't get indexed unless you sync it to the client and index it manually.

(And if you have file syncing set to "as needed", you wouldn't even have the file, so it wouldn't be sufficient to kick off indexing automatically on a synced computer. The PDF should be indexed server-side and full-text content should be synced.)