CYB-329 Gallery Performance: Don't retrieve the vimeo thumbnail every time.

* Created a new worker that does the processing for urls submitted.
* no specs yet, to be written for followup PR

https://jira.ets.berkeley.edu/jira/browse/CYB-329
Merge pull request #201 from coyote/thumbnails_on_media_url

CYB-329 Gallery Performance - cache thumbnail URL