#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

store/sql_post_store_test.go

@@ -1661,3 +1661,37 @@ func TestPostStoreGetPostsBatchForIndexing(t *testing.T) {
 		}
 	}
 }
+
+func TestGetLastPostForChannel(t *testing.T) {
+	Setup()
+
+	o1 := &model.Post{}
+	o1.ChannelId = model.NewId()
+	o1.UserId = model.NewId()
+	o1.Message = ""
+	o1.Type = model.POST_JOIN_CHANNEL
+
+	o1 = (<-store.Post().Save(o1)).Data.(*model.Post)
+
+	if r1 := <-store.Post().GetLastPostForChannel(o1.ChannelId); r1.Err != nil {
+		t.Fatal(r1.Err)
+	} else {
+		if r1.Data.(*model.Post).Id != o1.Id {
+			t.Fatal("invalid returned post")
+		}
+	}
+
+	o2 := &model.Post{}
+	o2.ChannelId = o1.ChannelId
+	o2.UserId = model.NewId()
+	o2.Message = ""
+
+	o2 = (<-store.Post().Save(o2)).Data.(*model.Post)
+	if r2 := <-store.Post().GetLastPostForChannel(o2.ChannelId); r2.Err != nil {
+		t.Fatal(r2.Err)
+	} else {
+		if r2.Data.(*model.Post).Id != o2.Id {
+			t.Fatal("invalid returned post")
+		}
+	}
+}