0.19.1 production release

• The API will hide almost all(*) dead links from the search results. #137
• "foreign_identifier_url" and "creator_url" URLs will always return valid URLs, even if our database contains flawed upstream data. #147
• Re-enable thumbnail serving now that better ones are available. cc-archive/cccatalog-frontend#118
• Assorted performance and cost optimization improvements.

(*) Known bugs:

• In some very rare situations, Flickr will delete a thumbnail but not the full-sized image from their servers. This results in a broken thumbnail being displayed in CC Search, but clicking on the broken thumb will reveal that the image does in fact still exist.
• Images which do not support HTTPS cannot be loaded on the front end but still show up in the results. We are considering producing our own thumbnails for these images in the future.