Add "Nodes" view to the Admin UI "Cloud" tab

The various sub-tabs of the "Cloud tab" were designed before anyone was making real use of SolrCloud, and when we didn't really know the use-cases we would need to support. I would argue that, whilst they are pretty (and clever) they aren't really fit for purpose (with the exception of tree view).

Issues:

• Radial view doesn't scale beyond a small number of nodes/collections
• Paging on the graph view is based on collections - so a collection with many replicas won't be subject to pagination
• The Dump feature is kinda redundant and should be removed
• There is now a major overlap in functionality with the new Collections tab

What I'd propose is that we:

• promote the tree tab to top level
• remove the graph views and the dump tab
• add a new Nodes tab

This nodes tab would complement the collections tab - showing nodes, and their associated replicas/collections. From this view, it would be possible to add/remove replicas and to see the status of nodes. It would also be possible to filter nodes by status: "show me only up nodes", "show me nodes that are in trouble", "show me nodes that have leaders on them", etc.

Presumably, if we have APIs to support it, we might have a "decommission node" option, that would ensure that no replicas on this node are leaders, and then remove all replicas from the node, ready for it to be removed from the cluster.