CYB-321 - Provide UI support for generic URL submission

https://jira.ets.berkeley.edu/jira/browse/CYB-321

Building on top of https://github.com/ets-berkeley-edu/datacultures/pull/222

- Added UI support for generic URL assignment submission
- Added a default thumbnail for assignment submissions without a thumbnail

Note that the default thumbnail would ideally not be an image, but an HTML/CSS/FontAwesome only thumbnail. However, as this requires a structural HTML change, I've opted to go for an image approach to ensure that no regressions are introduced at this point.
@nicolaasmatthijs use Illustrator instead - you can install it on https://software.berkeley.edu/adobe. Also make sure you click on "export to outline" when you save the SVG file.

These are the export options we usually use:
https://docs.google.com/presentation/d/1CNQLbqC0krocy_fZrM5fZ-YmQ2JgEADRh3qR6RbOOGk/edit#slide=id.g2762eebf0_210

Definitely let me know if you have any more questions.