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.

Change-Id: I2d9c34a1ceb8c92e8ff813301186a2f5c9f499b4
Related-To: https://bugzilla.redhat.com/1064749
Signed-off-by: Mike Kolesnik <mkolesni@redhat.com>