Extract audio URL prefixes into site setting

[rpeterman-gp]

When prefixes of audio URL's are changed in Dovetail, all URI's for audio become out of sync. Instead of updating the URI of all audio media posts, we could store the newest prefix found during episode import as a site setting. The frontend could then use that setting to update the prefix portion of the audio URL at the time of playback.

This doesn't address changes to Dovetail audio file path (like when we added the _), but will make sure the frontend will play audio with the most recent set of tracking prefixes, which could be important if any past tracking services go away (chartable) that could break URL's.