(Bug 27559) Preserve tab selection after submit in [[Special:Preferences]]

Firstly it was planned to be implemented via cookies, but they behave terribly.
So, for now it's a HTML5-only solution (via sessionStorage).

Change-Id: I7b7f14a6dca580a4498bda1ada48d1a3ba7c69c3
Merge "(Bug 27559) Preserve tab selection after submit in [[Special:Preferences]]"