Make the CMS load channels from Firebase as fallback

[oskarrough]

We can't spend more time on the technology stack we're using on github.com/internet/radio4000, which is live on radio4000.com Before this project can replace what's online on radio4000.com it should:

• [x] All existing channels can still be viewed and played on their regular URLs
• [x] All existing channels are "read only"
• [x] You can create a new Supabase user and migrate your own channel by signing in to Firebase and triggering a migrate function (outside of this repo)
• [x] You can't reuse a Firebase channel slug for a new Supabase channel. Could hardcode the current slugs.

Is this correct? What else?

[oskarrough]

Seems to work now. Although the slug check is only in our frontend