Support link to HTML to PDF

[bganglia]

Links like https://doi.org/10.1016/j.jns.2020.116832 do not lead directly to a PDF - they go through HTML first. This leads to failures when loading PDF handles / saving PDF files.

This was separated from #27

In the example above the key element is this:

<input type="hidden" name="redirectURL" value="http%3A%2F%2Fjns-journal.com%2Fretrieve%2Fpii%2FS0022510X20301684" id="redirectURL"/>