Share custom message formats between notification rule templates

Allow custom message formats to be shared between notification rule templates that live on the same element template and/or utilize the same event frame template. Example: An event frame template specifies an event frame attribute that will vary between several integer values. Several Notification rule templates can be configured on the same element that trigger on the creation of event frames using this event frame template. Each Notification Rule template will trigger on different values of the event frame attribute. Each Notification Rule template should use the same message format, but will specify a different set of subscribers. Currently, the same message format has to be manually recreated for each notification rule template, as custom message formats are not shared between notification rule templates. Note: Global message formats cannot be used, as the message format uses the attribute value from the event frame template.