#4755 Combining consecutive user join/leave system messages to single message and few other changes. (#5945)

fix 7 and 8

remove @ at "{username} joined the channel"

refactor and update test

api4/post_test.go

@@ -359,7 +359,7 @@ func TestUpdatePost(t *testing.T) {
 		t.Fatal("failed to updates")
 	}
 
-	post2 := &model.Post{ChannelId: channel.Id, Message: "zz" + model.NewId() + "a", Type: model.POST_JOIN_LEAVE}
+	post2 := &model.Post{ChannelId: channel.Id, Message: "zz" + model.NewId() + "a", Type: model.POST_HEADER_CHANGE}
 	rpost2, resp := Client.CreatePost(post2)
 	CheckNoError(t, resp)