webadmin: Face lift for Discovery popup

Mostly improvements to HorizontalSplitTable widget; improved behaviour
upon items change, changed button styling, added table borders, added
customizable table headers. Also changed a couple of network-specific
text labels.

Change-Id: I247df809b2cf21488f0b6c3109294c24cb3db914
Signed-off-by: Lior Vernia <lvernia@redhat.com>