[FIXED JENKINS-12334 JENKINS-12446 JENKINS-12650]

Bundle slf4j binding to the war.
See the comment in war/pom.xml for detailed discussion.

This is fundamentally a "damned if I do, damned if I don't" situation,
but given that JENKINS-12334 is a fatal error, and the downside of
bundling the binding jar is "multiple binding" warning, it seems like
the lesser evil is to bundle it and risk some warnings.