igloo: open the new designer from both custom graph pages

Both custom graph pages render the Vue designer now, still honouring
"graph_id" so saved links survive. The service metrics painter keeps
offering a metric to a new forecast graph, but its custom graph entries
are gone: the designer picks a host, service and metric itself rather
than taking one handed to it.

CMK-37237

Change-Id: I1b7bad684c65262a34956184125a7e65a8ad1225