[PLT-1015] Make all Mattermost system emails Multi-Part, with both a HTML and Plain Text version. (#5698)

* Implementation to Make all Mattermost system emails Multi-Part, with both a HTML and Plain Text version

* update lib

* update code per review

* update to use the mattermost repo

app/email_batching.go

@@ -218,7 +218,7 @@ func sendBatchedEmailNotification(userId string, notifications []*batchedNotific
 	body.Props["BodyText"] = translateFunc("api.email_batching.send_batched_email_notification.body_text", len(notifications))
 
 	if err := utils.SendMail(user.Email, subject, body.Render()); err != nil {
-		l4g.Warn(utils.T("api.email_batching.send_batched_email_notification.send.app_error"), user.Email, err)
+		l4g.Warn(utils.T("api.email_batchings.send_batched_email_notification.send.app_error"), user.Email, err)
 	}
 }