LMS URL assignments made from via.hypothes.is/ links show the web app and not the LMS app

**Describe the bug**
When creating an LMS assignment, using a link like https://via.hypothes.is/https://www.npr.org/sections/coronavirus-live-updates/2020/09/04/909433912/job-gains-slowed-again-in-august-as-employers-add-1-4-million-jobs causes the LMS assignment to show the web app instead of the LMS app.


**To Reproduce**
Steps to reproduce the behavior:
1. Log into an LMS with Hypothesis installed, go to make an assignment
2. Select the URL option and use https://via.hypothes.is/https://www.npr.org/sections/coronavirus-live-updates/2020/09/04/909433912/job-gains-slowed-again-in-august-as-employers-add-1-4-million-jobs
3. Save/publish assignment and see the Public group in the Sidebar

**Expected behavior**
When using a Via link with the URL option you should not end up with an LMS assignment that opens the web app. This could be preventing saving the link, stripping out the Via portion, etc.

**Additional context**
https://hypothesis.zendesk.com/agent/tickets/12997
In this ticket the instructor didn't know they were adding the Via link, but the LMS URL had via embedded in it. to use the above URL, the differences between the Via URL and the non-Via URL are:
- https://lms.hypothes.is/lti_launches?url=https%3A%2F%2Fvia.hypothes.is%2Fhttps%3A%2F%2Fwww.npr.org%2Fsections%2Fcoronavirus-live-updates%2F2020%2F09%2F04%2F909433912%2Fjob-gains-slowed-again-in-august-as-employers-add-1-4-million-jobs
- https://lms.hypothes.is/lti_launches?url=https%3A%2F%2Fwww.npr.org%2Fsections%2Fcoronavirus-live-updates%2F2020%2F09%2F04%2F909433912%2Fjob-gains-slowed-again-in-august-as-employers-add-1-4-million-jobs

I could imagine an instructor getting to this point by:
1. Create an assignment using a normal URL
2. Clicking on a link in that assignment to open a new page
3. The new page opens in Via and the instructor does not understand that this has changed the URL
4. The instructor copy/pastes the Via URL to create a new assignment.

Start by reproducing the assignment flow with a via.hypothes.is URL and inspect the lti_launches URL shown in the issue. Compare it with the non-Via URL and verify that a saved assignment opens the LMS app rather than the web app; the issue lists possible handling such as rejecting or stripping the Via portion.

Written by the indexing model from the issue text.