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.
I haven't discussed this with @elicochran yet, but given that URLs are often very long and could take up multiple lines of screen real estate, I was thinking that just offering an `Open` might be a cleaner approach, especially as they'll see a screenshot of the page below the message. @elicochran : Any thoughts?