Prepopulate VitalSource chapter selection from pasted URL

The VitalSource assignment picker allows users to paste a link from the VitalSource book viewer, from which we extract the book ID. It would be useful if we could also extract the CFI and then auto-select the appropriate chapter in the chapter list. Currently the teacher always has to pick the chapter they want from the chapter list, even if we could figure it out from the URL.

On a call on 2022-08-18 we also discussed a potential need to allow the VS book picker to be used to configure an assignment even when we can't query the specific book/chapter with the VS API keys that we have. In that case we want to preserve the CFI from the pasted link in the `vitalsource://` URL that we generate for the assignment.