New cluster row in Grafana Dashboard

Added a new row to the default Grafana dashboard. Also uploaded to https://grafana.com/grafana/dashboards/12456

The new row shows

• Number of total and live nodes in cluster
• Number of collections
• Number of replicas, and number of replicas in each state
• Who is the shard leader of each shard (hover to see core-name)
• Historic replica state (hover to see state code)
• All the new panels (except live-nodes) react to the template dropdowns on top, so you can filter by node or collection and then view replicas or leaders.

Also updated URLs to new Solr site.

For thereplica state gauges to work, I also had to add the 'state' label to the solr_collections_replica_state metric in the exporter.