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 this is really interesting, I don't think we ever had to use `clip-path` before.

This is what I get when I remove the empty layers / groups in Illustrator:
```
<?xml version="1.0" encoding="utf-8"?>
<svg version="1.1" id="Layer_1" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" x="0px" y="0px"
	 viewBox="-205 296 201 201" enable-background="new -205 296 201 201" xml:space="preserve">
<path fill="#D1D2D2" d="M-66.5,367.6c1,1,1.9,2.4,2.6,4.1c0.7,1.7,1.1,3.3,1.1,4.7v61.7c0,1.4-0.5,2.6-1.5,3.6s-2.2,1.5-3.6,1.5
	H-140c-1.4,0-2.6-0.5-3.6-1.5s-1.5-2.2-1.5-3.6v-85.7c0-1.4,0.5-2.6,1.5-3.6s2.2-1.5,3.6-1.5h48c1.4,0,3,0.4,4.7,1.1
	c1.7,0.7,3.1,1.6,4.1,2.6L-66.5,367.6z M-69.7,436.4v-54.9H-92c-1.4,0-2.6-0.5-3.6-1.5s-1.5-2.2-1.5-3.6v-22.3h-41.2v82.3H-69.7
	L-69.7,436.4z M-88.1,355.7c-0.4-0.4-1.2-0.8-2.2-1.2v20.1h20.1c-0.4-1-0.8-1.8-1.2-2.2C-71.4,372.4-88.1,355.7-88.1,355.7z"/>
</svg>
```