SPIKE: Fix previously-saved "PDF.js" doc titles so when PDFs are re-used in the LMS app we capture a better file name

[mkdir-washington-edu]

Describe the bug
Title isn't great. Not trying to propose a solution. Slack convo here: https://hypothes-is.slack.com/archives/DLX8FBWBB/p1621610716002300

We're going to be collecting better titles than "PDF.js" in the future on the occasions where there is no "title" PDF metadata: hypothesis/client#3372.

This will not fix documents for which we've already stored "PDF.js" as the file title. As an example, a professor is probably going to re-use the same PDF every semester, so they'll continue to get "PDF.js" as the doc title going forward. We'll want to find a way to fix already saved document titles.

Expected behavior
Using a PDF with no "title" metadata, even one you've used before, results in a better title than "PDF.js".

Additional context
These titles are especially important in Notebook in the LMS app where it can be difficult to tell which PDF.js assignment an annotation is associated with.

[mkdir-washington-edu]

List of documents where doc title = "PDF.js viewer" - GDrive link

Some example documents:

• Summer Institute 2020
• Mulch Product Information Sheet
• Mixed High-Order Attention Network for Person Re-Identification
• PARTICIPANT GUIDE
• INTRODUCTION TO ENVIRONMENTAL HISTORY
• Becoming American, Being Indian
• Rhetorics of Display

Next steps:
Support: Test to see if new annotation updates the title (doc titles visible in notebook)
Then delete one doc title and see if it fixes
Then TBD