CYB-321 Use Canvas' thumbnailing for non-YT/Vimeo URLs

* Adds a new model, GenericUrl
* Adds queries for #index and #show actions of the GalleryController

[CYB-321  Use Canvas' thumbnailing for non-YT/Vimeo URLs](https://jira.ets.berkeley.edu/jira/browse/CYB-321)
Comment from @christianv : 

could we format this file? (2 spaces - jsonlint)

```
{
  "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"
    }
  ]
}
```