Allow messages from different channels to be combined

[renbinden]

• Add a configuration setting that sets groups of channels that can have their messages combined
• For each recipient of a message, if subsequent messages in a batch are all part of the same group, group all formatted messages into one message
• Keep accumulating messages until one is found that is not in the group, then start another group.

[renbinden]

Had a crack at this, it's not super feasible given that the undirected pipeline would need to be run for each part and the directed pipeline would need to be run once for each part for each player. Radius filters also make it very difficult since different messages may not reach all recipients, so groups may affect different things. Going to close it for now.