ui: dashobard/DataStoreStatistics: add 'Available' column

for some storages, it is valuable information, e.g. if one has datastores
on separate datasets of the same zpool

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

www/dashboard/DataStoreStatistics.js

@@ -72,6 +72,13 @@ Ext.define('PBS.DatastoreStatistics', {
 	    width: 90,
 	    renderer: Proxmox.Utils.format_size,
 	},
+	{
+	    text: gettext('Available'),
+	    dataIndex: 'avail',
+	    sortable: true,
+	    width: 90,
+	    renderer: Proxmox.Utils.format_size,
+	},
 	{
 	    text: gettext('Usage %'),
 	    dataIndex: 'usage',