[JENKINS-21190] - lib/layout/layout.jelly: Interpret norefresh=="false" as "autorefresh is enabled"

The fix allows to explicitly disable automatic page refreshes in the @View extensions.
Resolves https://issues.jenkins-ci.org/browse/JENKINS-21190

Signed-off-by: Oleg Nenashev <o.v.nenashev@gmail.com>