Improve permission checking for chat broadcast subscribers

[dktapps]

Description

Currently, the chat broadcast system relies on each recipient to do permission checks.

This is currently done in Player in a very janky way by (un)subscribing to the needed channels when permissions are updated.

This proposes that:

• Any CommandSender can subscribe to a broadcast channel, irrespective of permissions
• However, broadcastMessage() should check if the subscriber has the permission associated with the channel in question

Justification

More consistent behaviour. If more types of CommandSender are added by plugins (e.g. RCON) and subscribed to these channels, their permissions won't be enforced, which could lead to data being leaked. This is sort of similar to the easy security issues caused by having commands check their own permissions rather than the server doing it.

Alternative methods