ui: add datastore usages to dashboard

shows an overview over the datastores, including a small chart of the
past month and an estimation of when its full

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>

www/Dashboard.js

@@ -193,6 +193,10 @@ Ext.define('PBS.Dashboard', {
 		}
 	    ]
 	},
+	{
+	    xtype: 'pbsDatastoresStatistics',
+	    height: 250,
+	},
 	{
 	    xtype: 'container',
 	    height: 250,