[PLT-5170] Add join the channel system message for the person who created the channel (#7299)

* add join the channel system message for the person who created the channel

* update test

api/admin_test.go

@@ -246,7 +246,7 @@ func TestGetTeamAnalyticsStandard(t *testing.T) {
 			t.Fatal()
 		}
 
-		if rows[2].Value != 6 {
+		if rows[2].Value != 9 {
 			t.Log(rows.ToJson())
 			t.Fatal()
 		}