This PR adds a new [extension point][] to each row of the
alert instances table. This allows plugins to add actions
to a dropdown menu in the rightmost column of the table.
Those actions are passed the alert instance so can use it
for contextual handling.
See https://github.com/grafana/machine-learning/pull/3461
for an example of how this can be used (e.g. by Grafana Sift
here).
Ben Sully