Automatically download open-access PDFs when saving via the connector

If there's no translated PDF or the translated PDF fails and the item
has a DOI, check Zotero's Unpaywall mirror for possible sources and try
to download one of those.

Unlike with "Add Item by Identifier" and "Find Available PDF" in the
item context menu, this does not try the DOI/URL page, since it would
result in more data leakage and most of the time you'd be saving from
the DOI page already. We could consider offering it as an option, but
for it to be useful, you'd have to have an institutional subscription,
be on-campus or connected via VPN (for now), and be saving from
somewhere other than the main page.

A new connector endpoint, sessionProgress, takes the place of
attachmentProgress. Unlike attachmentProgress, sessionProgress can show
new attachments that have been added to the save, and with a little more
work should also be able to show when a parent item has been recognized
for a directly saved PDF.

This also adds support for custom PDF resolvers, available to all PDF
retrieval methods. I'll document those separately.

Closes #1542

resource/jspath.js

@@ -0,0 +1 @@
+../node_modules/jspath/lib/jspath.js