Bug 840716: Download thumbnail image instead of storing URLs.

- Adds an admin command to download thumbnails for existing videos.
- Alters the approval processing to happen synchronously on save instead
  of asynchronously via celery to help lower the risk of making a video
  public before it is fully processed.
- Move approval email outside of process_approval to avoid emailing the
  user before the video changes have completed and been saved 
  successfully.
- Bug 840717: Only show pagination if there is more than one page of
  results.
- send_approval_email now uses the default locale if the user doesn't
  have a locale or profile.
- Improve test patching to avoid contacting outside services during
  tests.