webadmin: Date formatting when deleting a snapshot

When deleting a VM's snapshot a formatted date is now shown based on a
constant taken from another component in the system (DiskGeneralModel)
for system consistency. this constant is intended to be exported to
some util class later on.

Bug-Url: https://bugzilla.redhat.com/1158959
Change-Id: I5d6a3f43557696fd35256b4a3fc55a5fbf59fdba
Signed-off-by: Amit Aviram <aaviram@redhat.com>