Add warning when unsubscribing from private channel using toggle

[FeldrinH]

When unsubscribing from a private channel using the unsubscribe option in the three dot menu, the app shows a confirmation dialog with a warning that you might not be able to rejoin.

However, when unsubscribing using the toggle switch next to the channel name, this warning is not shown. This makes it very easy to accidentally unsubscribe from a private channel by hitting this toggle.

I think the toggle switch should show the same warning that the unsubscribe option in the menu does.

Alternatively, maybe private channels shouldn't have this toggle switch at all, since the convenience of fast subscribe-unsubscribe doesn't really benefit private channels where you can't subscribe yourself.

[FeldrinH]

Screenshot to clarify what I'm talking about:

[alya]

Warning here when we'd warn from the other UI makes sense to me.