core+webadmin: Fix aggregate unlimited quota

Unlimited specific quota was not aggregated to unlimited.

This was fixed both in QuotaManager and in the WebAdmin.

Added a pop-up confirmation window alerting the user that using
unlimited specific quota is inadvisable and asking his confirmation.

Change-Id: If8f5eb7b2ad1fd5264039ede728c746b0a3cf1b3
Bug-Url: https://bugzilla.redhat.com/923868
Signed-off-by: Ofri Masad <omasad@redhat.com>