Decouple emailservice from app package (#17827)

* decouple emailservice from app package

* fix some escaped errors

* move email package under app directory

* fix i18n

* reflect review comments

app/plugin_api.go

@@ -780,7 +780,7 @@ func (api *PluginAPI) SendMail(to, subject, htmlBody string) *model.AppError {
 		return model.NewAppError("SendMail", "plugin_api.send_mail.missing_htmlbody", nil, "", http.StatusBadRequest)
 	}
 
-	if err := api.app.Srv().EmailService.sendNotificationMail(to, subject, htmlBody); err != nil {
+	if err := api.app.Srv().EmailService.SendNotificationMail(to, subject, htmlBody); err != nil {
 		return model.NewAppError("SendMail", "plugin_api.send_mail.missing_htmlbody", nil, err.Error(), http.StatusInternalServerError)
 	}