Add websocket event and cache invalidation for deleting channels (#2807)

model/message.go

@@ -13,6 +13,7 @@ const (
 	ACTION_POSTED             = "posted"
 	ACTION_POST_EDITED        = "post_edited"
 	ACTION_POST_DELETED       = "post_deleted"
+	ACTION_CHANNEL_DELETED    = "channel_deleted"
 	ACTION_CHANNEL_VIEWED     = "channel_viewed"
 	ACTION_NEW_USER           = "new_user"
 	ACTION_USER_ADDED         = "user_added"

model/post.go

@@ -16,6 +16,7 @@ const (
 	POST_SYSTEM_GENERIC        = "system_generic"
 	POST_JOIN_LEAVE            = "system_join_leave"
 	POST_HEADER_CHANGE         = "system_header_change"
+	POST_CHANNEL_DELETED       = "system_channel_deleted"
 	POST_EPHEMERAL             = "system_ephemeral"
 )