Do not unsubscribe to channel realtime events on switching channels

[nikkothari22]

As of now:

1. A user opens a channel -> we subscribe to real-time events for the channel.
2. User switches channel -> we unsubscribe to real-time events for the channel
3. Repeat

This is pretty expensive since the socketio server then makes a call to can_subscribe_doc every time we switch channels.

Instead, we should subscribe to a channel once and then not unsubscribe until either the user leaves the channel / the channel is deleted.

The only problem here is that if the user subscribes to a channel, they receive new messages in websocket events. If this user is then removed from the channel, we can't unsubscribe them from the websocket channel and they would still keep getting the new messages. Obviously this won't be shown on the UI, but might still be considered a risk?

[linear]

RAV-139 Do not unsubscribe to channel realtime events on switching channels