This commit updates the stale logic in 'toggle_topic_visibility_policy'
function. It was not updated after the addition of 'follow' visibility
policy resulting in buggy behavior.
Earlier, 'Shift + M' hotkey was working in unsubscribed channels.
This commit updates the logic to make it non-functional. This
helps to keep the UI consistent as we don't offer a way to
change visibility_policy in unsubscribed channels via
recipient_bar_controls or other UI elements.
Note: We don't implement this restriction in server because
we don't delete the user-topic states in db when a channel is
unsubscribed as it helps to retain those states when re-subscribed.
And implementing this restriction at server level can create
a confusion of why such states exists in db when API doesn't allow.
Prakhar Pratyush