webadmin: Extract external subnet model & widget

Extracted the representation & functionality of external subnet to a
model & widget in order to facilitate code reuse and DRY.

Conflicts:
	frontend/webadmin/modules/webadmin/src/main/java/org/ovirt/engine/ui/webadmin/section/main/view/popup/AbstractNetworkPopupView.java

Change-Id: I2d9c34a1ceb8c92e8ff813301186a2f5c9f499b4
Bug-Url: https://bugzilla.redhat.com/1064749
Signed-off-by: Mike Kolesnik <mkolesni@redhat.com>
(cherry picked from commit b9a64c8b3827fe66cbfbac57581a73f181cc6cc7)