Consistent license message for all the go files (#13235)

* Consistent license message for all the go files

* Fixing the last set of unconsistencies with the license headers

* Addressing PR review comments

* Fixing busy.go and busy_test.go license header

config/migrate_test.go

@@ -1,11 +1,15 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
 package config
 
 import (
 	"fmt"
+	"testing"
+
 	"github.com/mattermost/mattermost-server/v5/testlib"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
-	"testing"
 )
 
 func TestMigrateDatabaseToFile(t *testing.T) {