Retraction Watch integration

- Check for retracted items using data from Retraction Watch
- Show an X next to retracted items in the items list, and show a
  scary message at the top of the item pane with more info and links.
- Lookup is done in a privacy-preserving manner using k-anonymity --
  the server is unable to determine the specific items that exist in
  the client, so people who don't sync don't need to share any library
  data (though the server doesn't log the lookups anyway).

TODO:

- Pop up an alert when new items are found
- Show a confirmation prompt when citing a retracted item
- Support items without DOIs or PMIDs
- Add a proper PMID field and expand DOI to more item types so these
  values don't need to be parsed out of Extra
- Clear the banner immediately when all possible fields are cleared
  instead of waiting a few seconds

components/zotero-service.js

@@ -110,6 +110,7 @@ const xpcomFilesLocal = [
 	'quickCopy',
 	'recognizePDF',
 	'report',
+	'retractions',
 	'router',
 	'schema',
 	'server',