webadmin: Disks and Snapshots sub-tabs resizable columns

Disks and Snapshots sub-tabs now support
resizable columns.

* Invoked enableColumnResizing() on each sub-tab.
* Added a static width value for each column.

Change-Id: Ief60915ae0d7ddbfe5d9cb808756796773478541
Signed-off-by: Daniel Erez <derez@redhat.com>