MM-54238 Add WebSocket broadcast hook and don't broadcast when other users were mentioned (#24641)

* MM-54238 Initial implementation

* MM-54238 Move websocket hook into app package

* MM-54238 Add tests for mentions in posted websocket messages

* Fix styling

* Fix other styling

* Idiomatic ID naming for new code

* Fix more styles

* Separate hooks to add mentions and followers

* Improved error handling for invalid types in hooks

* Rename HasChanges to ShouldProcess

* Pass broadcast hooks through hubStart

* Add test helper for asserting json unmarshaling

* Fix missing arguments in tests

* Ensure broadcast hooks are sent across the cluster and not to users

* Ensure tests actually cover following a post

* Fix code broken by merge

* Go vet again...

* Deep copy event before processing it with hooks

* Replace RemoveBroadcastHooks with WithoutBroadcastHooks

* Address feedback

* Add helper to fix type information for hook args

* Wrap WebSocketEvent and simplify BroadcastHook

* Address feedback

* Address feedback

server/channels/app/platform/service.go

@@ -344,8 +344,8 @@ func New(sc ServiceConfig, options ...Option) (*PlatformService, error) {
 	return ps, nil
 }
 
-func (ps *PlatformService) Start() error {
-	ps.hubStart()
+func (ps *PlatformService) Start(broadcastHooks map[string]BroadcastHook) error {
+	ps.hubStart(broadcastHooks)
 
 	ps.configListenerId = ps.AddConfigListener(func(_, _ *model.Config) {
 		ps.regenerateClientConfig()