[EBSCOhost] Update PDF scraping. Also detect pdfviewer pages.

Please take a quick look. Should be ok to merge.

I kept the old ways of detecting PDF url in case the EBSCOhost update hasn't been deployed to all users (or something else happens). The disadvantage of the new method (while more reliable) is that we use processDocuments instead of doGet, so it would take longer to load the intermediate page.

Also, I added support for pdfviewer page (the page that displays the PDF).

I removed some PDF URL "cleanup". Some of it is no longer relevant with processDocuments and I can't exactly remember why others are there to start with (e.g. replacing accession number), since whatever URL we fetch from the page, should be the URL where the PDF is located. Wish I noted a test case...