When unsubscribing from a channel, check if org would lose content access

[chrisbobbe]

Reading web-app code, it looks like web gives a message like

Your organization will lose access content in this channel, and nobody will be able to subscribe to it in the future.

when you're about to unsubscribe from a private channel where you're the only subscriber and nobody has permission to subscribe or add subscribers.

We should do the same.

References

• Prerequisite issue: Implement group-based permission checks #814
• Overlapping issue: Check group-based permission to subscribe to a channel, as needed #1786

[gnprice]

A prerequisite for this is #814 group-based permissions. This might be one of the things that's easy to sweep through once we have that in place.

(Mentioning it here partly so that there'll be a backlink from that issue.)

[gnprice]

… Oh and I guess a more specific prerequisite is:

• Check group-based permission to subscribe to a channel, as needed #1786

Or parts of that are a prerequisite, anyway.