mgr/dashboard: Add the Prometheus alerts

mgr/dashboard: Add the Prometheus alerts

The backend is now capable of receiving alert notifications from
the Prometheus alertmanager and it can get all alerts with all kinds of
parameters from the API of the same.

In the frontend Prometheus alerts can be found in "Cluster > Alerts". Incoming
notifications can be seen as usual in the notifications popover.

To clarify:
Prometheus alerts are received from the alertmanager API.
Prometheus alert notification are send from the alertmanager to the
backend receiver. An alert notification can have multiple alerts, but
these alerts differ from the prometheus alerts.

To clarify that, I've added some models and services.

If one of the methods to get alerts contains changes the user will be
notified.

The documentation explains how to configure the alertmanager to use the
dashboard receiver and how to connect the use of the alertmanager API.
Further it explains where to find the alerts and what happens if they
are configured and something is happening.

Fixes: https://tracker.ceph.com/issues/36721
Signed-off-by: Stephan Müller <smueller@suse.com>