Add caching headers to logos + old URLs

[pushcx]

Following lobsters/lobsters#1325 and lobsters/lobsters#1412, @viraptor noted that the touch icons don't all have caching headers. These are configured in roles/nginx/files/production/nginx/sites-available/lobste.rs and you should check that both the current urls (/touch-icon-144.png) and old urls I recently symlinked (/apple-touch-icon-144.png) all have the correct headers.