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.

The switch to justified is more finger friendly on tablets anyway,
even when not stacked.

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 RTL added for Bootstrap upstream in
anticipation of using CSSflip.
Merge pull request #143 from ds125v/justify_navs

use nav-justified for responsive tabs
admin search: use placeholder text & option-group

use the Bootstrap styles for an option-group to
connect the search button and text entry field.

Stops weird word-wrapping things with the button.

Not 100% sure on the accessability implications of
using placeholder instead of a label, could add a
sr-only label back too. Though the info I could
find suggested that adding more labels would confuse
browsers more, while just placeholder worked better.

Not compatible with IE8.
course search: add the form-control class

Needed to make the text entry look nice.

Similar comment about the use of placeholder to the last commit.
MDL42634: opacity causes trouble for dropdowns

Fix for a bug that's currently being worked on in core.

Opacity is a bit weird, so replace it with some lighten'd
colors using less color functions.
various tidy ups, add newlines, plus compile less
Merge pull request #144 from ds125v/blocks

Blocks