Import thumbnails via API

Created by: rjw57

We should import thumbnails via the JWPlayer API. It's a little non-trivial since this would also require having some way of determining when we have set the thumbnail for a video. This could be via a custom "has_custom_thumbnail" prop or similar.

1. Store in the custom tags in jwplayer: SHA256 of the uploaded thumbnail
2. check if the last_updated is different and do a diff of the SHA256 of the thumbnails
3. upload the new one if that is the case