webadmin,userportal: Fix Maven build-time warnings

1. gwt-extension POM now uses ${project.version} instead
   of ${version} within the <finalName> declaration

2. frontend POM no longer duplicates plugin declaration
   for build-helper-maven-plugin -- both <execution>s
   are now placed under single plugin declaration

Change-Id: Id541ba5b01769dc5e8cab16f8260ea20b95e7270
Signed-off-by: Vojtech Szocs <vszocs@redhat.com>