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.
This is part of the PR that this PR build on top of. @coyote : Would you be able to incorporate this comment into #222 ?