Source/generator URL has misleading label

[mattbostock]

The 'generator URL' which links to a graph in Prometheus UI is currently labelled according to the Alertmanager instance.

This is misleading, since the Prometheus instance that triggered the alert does not necessarily map to a given Alertmanager.

Moreover, it makes it hard to find the link. Anecdotally, most people using Unsee don't seem to be aware that you can click on the name of the Alertmanager instance and see the data that triggered an alert.