CYB-308 Gallery video URLs: users ending up with two gallery items

Canvas' behavior to create a thumbnail for a URL submission and treat it like a file attachment was causing the Gallery to have a lot of thumbnail images.

  * Canvas does not process or store file attachments if there is a URL given anyway, so
     instead do not deal with attachemnts if there is a URL for the submission.

[CYB-308](https://jira.ets.berkeley.edu/jira/browse/CYB-308)