Show alert notification information

[stuartnelson3]

Show notification information on an alert. You can see when it last alerted, what receiver/s, and the result as alertmanager saw it, and time to next alert.

This could then be added to an alert page

[stuartnelson3]

Related to #172

[tzz]

A very useful view per alert would be "what do I call from amtool to create a replica of this alert?"