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/notification.go

@@ -517,12 +517,12 @@ func (a *App) SendNotifications(c request.CTX, post *model.Post, team *model.Tea
 		}
 	}
 
-	if len(mentionedUsersList) != 0 {
-		message.Add("mentions", model.ArrayToJSON(mentionedUsersList))
+	if len(mentionedUsersList) > 0 {
+		useAddMentionsHook(message, mentionedUsersList)
 	}
 
-	if len(notificationsForCRT.Desktop) != 0 {
-		message.Add("followers", model.ArrayToJSON(notificationsForCRT.Desktop))
+	if len(notificationsForCRT.Desktop) > 0 {
+		useAddFollowersHook(message, notificationsForCRT.Desktop)
 	}
 
 	published, err := a.publishWebsocketEventForPermalinkPost(c, post, message)