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 ideally this would be the hashed version of the image. The way we usually do this is by using a background image and `asset-data-url`. https://github.com/ets-berkeley-edu/calcentral/blob/96f5982560f624e707b98a277cadaeaaa5ce00be/app/assets/stylesheets/_icons.scss#L19

Or perhaps the back-end can return the hashed version in the `image_url` value.