Fix: inadvertent scroll when loading the instructor's extensions tab

This minor commit fixes an annoying UI bug with the individual due date extensions, [mentioned](https://github.com/edx/edx-platform/pull/4868#issuecomment-52525852) to me by @sarina in #4868. Whenever an instructor clicked on the "Extensions" tab, the browser would scroll down. This was due to a div having id "view-extensions", conflicting with the tabs script which sets location.hash to "view-[current tab name]" in order to track the current tab.

This was done for OpenCraft ( cc @antoviaque ) and falls under their organization contributor agreement.