Add missing format string and separators in event fingerprinting.
Add more required fields to Event.

This adds mandatory Summary and Description fields to Event.

As for the alert name, there were two options: keep it a separate field and
treat it separately everywhere (including in silence Filter matching), or
make it a required field in the event's labels. The latter was causing far
less trouble, so I went with that. The alertname label still doesn't have
a special meaning to most parts of the code, except that the API checks its
presence and the web UI displays it differently.
Implement PagerDuty notifications.
Minor UI cleanup.