Allow users to select a subset of a PDF for an assignment, like we do for VS books

[mkdir-washington-edu]

The problem
In conjunction with #1619, some instructors would like to use the same PDF for several assignments, treating each section o a PDF as the matter for that particular assignment. Like in our VitalSource book integration, this allows instructors using longer texts to make sure students stay focused on the specific part of the text required for the assignment.

It also saves instructors using long texts time and frees them from worrying about making mistakes (e.g. with splitting a PDF properly to get different fingerprints in each sub-pdf).

Example ticket: https://app.hubspot.com/contacts/6291320/record/0-5/18000785723

The solution
We could allow instructors the option of using a whole PDF or a page range. For PDFs with the appropriate tags we could also display the tag structure of the PDF, similar to how we show chapters in VS books.