icons after build completion.

Fix for [JENKINS-13195]

Adjust z-index for #search-box-completion to appear over the breadcrumb

Applied the change from Olav Reinert.

Make sure that we don't require Jenkins.getInstance() on CLICommand, since it can run on slaves.

JENKINS-13185: Fix Computer.getHostName() to return the host.name value from the slave rather than null

Fix for JENKINS-13185: If there is a fallback host.name specified, it should return that and not null.

This has a downside that we waste cache in user browser, especially for
plugin developers, but this provides the simplest migration for plugins,
and the same code works for pre 1.478 and post 1.478.