Backport MDL-35569 (critical JavaScript performance on dropdowns with IE8)

When using Moodle 2.3.x with IE8, a course page with a large number of sections (e.g. 52), editing turned on, and the activity chooser turned off, the JavaScript on the page takes around 40 seconds to run on a fast computer, causing the 'Stop this script?' dialog to pop up several times.

This is caused by the issue that MDL-35569 fixes. It should be possible to backport that fix to 2.3.x. Since we intend to do so in our live platforms, Andrew said he was planning to do so anyway, we figure other people might want it in general and it should go in the next core 2.3.x.