Check group-based permission to subscribe to a channel, as needed #1786
Description
We should check the relevant permission:
- before deciding to show a "Subscribe" button in the channel action sheet (Offer subscribe/unsubscribe in channel action sheet #1224)
- to give a specific warning "you won't have permission to resubscribe" in the confirmation dialog when unsubscribing from a channel (Offer subscribe/unsubscribe in channel action sheet #1224)
Help-center doc: https://zulip.com/help/configure-who-can-subscribe