webadmin: fix coverity issues

- Fixed potential divide by 0 in QuotaProgressBar.
- Fixed potential npe in PolicyUnitPanel.
- Fixed a couple of unchecked cast issues.

Change-Id: I7359a63d79a86dc2e6106befde4d7044abe499e6
Signed-off-by: Alexander Wels <awels@redhat.com>