TPv2 - add custom delivery service graph link

Add the ability to optionally display a graph icon on a delivery service page that links to a host responsible for displaying graphs for a delivery service. For example, the link may link to

https://the-place-where-grafana-runs.com?ds=xml-id

following this link will open a new tab. this is very similar to the link provided in the old TO UI that links to the value of the deliveryservice_graph_url parameter, however, TPv2 is unlike the old TO UI in the sense that it stores setting in traffic_portal_properties.json rather than in global TO parameters.