webadmin: Parameterize ImportNetworksModel and related classes

Parameterized generic instances of ListModel in ImportNetworksModel,
as well as the corresponding view and widget used in that view (and
nowhere else at the moment, which made things easier).

ExternalNetwork is changed to extend Model instead of EntityModel (as
it used no special capabilities of EntityModel).

Change-Id: I4d1e6e409edc794baa799807268755105f8e21fa
Signed-off-by: Lior Vernia <lvernia@redhat.com>