Story 831: Improve left nav

* Make left nav scrollable on smaller screens if it doesn't fit.
* Remove nav headings (new layout approved by Jared and Maryana).
* Make two bottom horizontal links text more dimmed (Jared's
  suggestion).
* Make menu narrower (fixed 15em on small devices, Jared's suggestion).
* Fix a slight misalignment of the blue left border of hovered menu
  items.
* Add shadow to the menu (consulted with Jared).
* Remove and refactor lots of CSS in the process.

Also:
* use mouseup fallback for closing the menu (touchend doesn't work in IE)
* don't close the menu when scrolling
* append toasts to #mw-mf-page-center instead of body to be consistent
  with drawers and avoid weird bugs
* set default background for body (#fff)

Changes tested on:
* Safari on iOS 6.0.1 and 6.1.2
* Android browser 4.2 and 2.3
* Chrome for Android (latest)
* Firefox for Android (latest)
* IE on Windows Phone 7.5
* latest desktop Chrome
* latest desktop Firefox
* Opera 14 for Android
* Opera Mini 7.5 for Android
* Nokia C3 browser

Bug: 49455
Change-Id: I670d81eb949b866472dde0498bf2402dda254214
Merge "Story 831: Improve left nav"