CYB-358 Process Youtube URLs

 * add specs for string_refinement
 * return empty thumbnail_url when a submitted URL is not YT or Vimeo
 * pass site_tag and site_id as string, not symbol
 * use regexp matching with anchor for efficiency
 * use #try to avoid undefined method on nil

[CYB-358 Process Youtube URLs](https://jira.ets.berkeley.edu/jira/browse/CYB-358)