Preserve Special:Preferences tab state across reload, open in new tab/window.

Preserve Special:Preferences tab state across reload, open in new tab/window.

* removed the preventDefault()s from handlers on the tab links, so the document hash gets updated (and thus preserved on reload)
* switched from #prefsection-N to #preftab-N on the generated links, so they don't cause the doc to jump down to the top of the tab sections on click now that the preventDefault is gone
* added a check for the hash after tab setup; if we have a #preftab-N then we invoke the tab-switch straight off.

This should keep devs from going insane when doing work on any preferences panel other than the first one