Move file backend to its own service (#9435)

* Move file backend to its own service

* Moving utils/inbucket to mailservice package

cmd/mattermost/commands/server.go

@@ -17,6 +17,7 @@ import (
 	"github.com/mattermost/mattermost-server/manualtesting"
 	"github.com/mattermost/mattermost-server/mlog"
 	"github.com/mattermost/mattermost-server/model"
+	"github.com/mattermost/mattermost-server/services/mailservice"
 	"github.com/mattermost/mattermost-server/utils"
 	"github.com/mattermost/mattermost-server/web"
 	"github.com/mattermost/mattermost-server/wsapi"
@@ -67,7 +68,7 @@ func runServer(configFileLocation string, disableConfigWatch bool, usedPlatform
 	}
 	defer a.Shutdown()
 
-	utils.TestConnection(a.Config())
+	mailservice.TestConnection(a.Config())
 
 	pwd, _ := os.Getwd()
 	if usedPlatform {