Use PostgreSQL for mapping IDs to article and sound URLs

[tobinus]

Currently, a local sqlite3 database is used. By centralizing this and using a PostgreSQL database, we avoid situations in which one instance thinks a URL is 404, while another one (correctly) knows what sound URL to redirect to. This can easily happen if a user fetches the feed itself from one instance, but uses another instance when fetching the episode (and that instance hasn't generated the feed yet).