CYB-308 Gallery video URLs: users ending up with two gallery items for one submitted item

  * 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.

https://jira.ets.berkeley.edu/jira/browse/CYB-308
Merge pull request #193 from coyote/thumbnails_for_videos_do_not_cache

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