use nav-justified for responsive tabs

I handcoded a responsive hack to make tabs switch between
normal tabs on wide screens and "stacked" tabs at small sizes.

I filed a bug with Bootstrap for them to do this in v3. They
closed that bug, but it turns out they added basically the same
thing as long as you use nav-justified.

http://getbootstrap.com/components/#nav-justified

The switch to justified is more finger friendly on tablets anyway,
even when not stacked, and in most situations it's not a radical visual change.

Nav pills also looks quite cool, someone should add a setting
for the theme to use those instead, it's just a swap of the class
name nav-tabs to nav-pills.

Oh, and I deleted some nearby RTL added for Bootstrap upstream in
anticipation of using CSSflip. Might be able to get rid of this whole file soon by moving the rest of the stuff to where it's actually relevant, rather than in this responsive ghetto.