Depending on launch URL, LTI 1.3 LineItems might have different URLs

[cmcfadden]

Currently we treat the lineitem as the unique identifier for an assignment. But it's possible to get two different lineitems for the same assignment, one with umn.instructure.com and one with canvas.umn.edu.

This won't cause any problems on the student side, but it could potentially cause an extra folder to be created in the Chime.

Easiest fix would be to mirror what we're doing with return URL and just force all line item URLs to a single domain. Would also need to write a migration to fix existing line items.