Show System Messages, Claim & JobInfo Updates

This adds shutdown messages when jenkins is shutting down or waiting for jobs to finish before restarting (JENKINS-20552). It also reads more information from the previous completed build, so that claim information will still be shown until the next build has completed, rather than jumping from claimed to unclaimed but building (JENKINS-19747).

To assist editing of this, the javascript has been moved into a separate file and several trivial javascript errors have been fixed. That was a separate commit, but seems to be bunched up together in this pull request, so might be easier to review from the history of this fork. If there's a good reason for keeping the js internal to the html file then it can be put back easily, just let me know.