Moving slash-commands out of app package (#14979)

* Moving slash-commands out of app package

* Fixing golint checks

* Fixing golangci-lint

* Fixing golangci-lint errors

cmd/mattermost/main.go

@@ -8,6 +8,9 @@ import (
 
 	"github.com/mattermost/mattermost-server/v5/cmd/mattermost/commands"
 
+	// Import and register app layer slash commands
+	_ "github.com/mattermost/mattermost-server/v5/app/slashcommands"
+
 	// Plugins
 	_ "github.com/mattermost/mattermost-server/v5/model/gitlab"