Finish Podcast support (#1828)

Finish Podcast support (#1828)

* fix: MusicController: Drop NOT NULL from publisher

The publisher column for Podcasts and Audio Books is allowed to be NULL,
update the database schema to allow this.

Signed-off-by: Eric B Munson <eric@munsonfam.org>
* feat: Subsonic: Finish podcast wireup

We still needed some changes to fetching stream details and the audio
stream itself to account for podcasts.

Signed-off-by: Eric B Munson <eric@munsonfam.org>
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Signed-off-by: Eric B Munson <eric@munsonfam.org>