MM-11230: Make permissions checks in commands failsafe. (#9392)

Also add additional unit tests to make sure the permissions tests are
completely solid.

app/command_channel_header_test.go

@@ -1,10 +1,14 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
 package app
 
 import (
 	"testing"
 
-	"github.com/mattermost/mattermost-server/model"
 	"github.com/stretchr/testify/assert"
+
+	"github.com/mattermost/mattermost-server/model"
 )
 
 func TestHeaderProviderDoCommand(t *testing.T) {