webadmin: Ignore FindBugs unconfirmed cast warning

Ignore all the unconfirmed casts in the gwt-common, uicommonweb and
webadmin  modules.

Unfortunately, these warnings are currently inherit to the design,
and handling them on an individual basis just adds noise to the
process.

Change-Id: I142db2592a9fe7b38a6751bcc362546ef46cc857
Signed-off-by: Allon Mureinik <amureini@redhat.com>