Web UI for editing content

[jcraigk]

In order to support users making corrections to the site (for exaple Forbins > Mockingbird narration cuts), add simple UI for admins only that supports uploading a new MP3 to replace a track.

[jcraigk]

We could add this as an admin option on a track context menu ("Replace audio file") that leads to a page where the new file can be uploaded. For anything more advanced than single-track audio file replacement (such as cutting or combining tracks), I think going to the CLI makes more sense.