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.
@christianv : How do you usually convert an OmniGraffle graphic with an embedded font into an SVG that can be embedded in the browser (without requiring the user to have that font)? Both Anne-Sophie and I are really struggling to find a setting tat works and I'm hesitant to spend more time on it.