mgr/dashboard: use service call instead of form component

mgr/dashboard: use service call instead of form component

For creating the silence from the notification sidebar, instead of using
the silence form which will require initializing the whole component on
the landing page, we can just call the prometheus service and pass on
the required data to the service call. This will fix showing the
`Prometheus not configured` error everytime we visit the landing page when
the prometheus is not configured

Fixes: https://tracker.ceph.com/issues/57576
Signed-off-by: Nizamudeen A <nia@redhat.com>