[JENKINS-10912] Use setInterval in refreshPart (#2539)

* [JENKINS-10912] Use setInterval in refreshPart

The current implementation of refreshPart creates a new closure every 5
seconds, which, in combination with XMLHttpRequest objects, results in a
significant memory leak in all major browsers. By using
window.setInterval to schedule periodic refreshes, only one closure per
id is created. Please see issue #10912 in the Jenkins tracker for
further details.

* Stop periodical calls if we can't find the div

* Don't check if isRunAsTest changed after page load