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 could we format this file? (2 spaces - jsonlint)

```javascript
{
  "assignment_id": 23908,
  "submitted_at": "2014-12-09T20:15:35Z",
  "url": "http://www.facebook.com/",
  "user_id": 90210,
  "attachments": [
    {
      "id": 5876223,
      "content-type": "image/png",
      "filename": "websnappr20141209-11859-1ledbdq.png",
      "url": "https://localhost:3100/files/259732101/download?download_frd=1"
    }
  ]
}
```