webadmin: Added Linq method containsByIdentity

To be used for checking whether a specific item exists in a collection
by reference (==) rather than by value (equals()).

Change-Id: I68361e35955879efe4185c5a47d6cbcb48fd1987
Bug-Url: https://bugzilla.redhat.com/1025288
Signed-off-by: Lior Vernia <lvernia@redhat.com>