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
restore block hiding CSS

I accidentally deleted this while working on hiding blocks from
students (.invisible) as opposed to this, which lets you hide
blocks from yourself (.hidden)

closes issue #147
Merge pull request #149 from ds125v/block_hiding

restore block hiding CSS
update bootswatches, add classic theme, reset
find-replace renderer for environment check table

I think this got lost in a merge because it's almost identical
to the renderer above it.

I really need to file a bug about the difficulty of overriding
these two renderers and add it here as a comment.
delete a few rules for non-existant tables

These rules are unecessary, either because the table doesn't
exist or look better without it.
buttons should look like button-default by default
use extend, not mixins for creating form-group row

Slightly less CSS, slightly more self-documenting
remove hardcoded color for login page details

This hardcode's #111 for a couple of things on the login page
which messes with bootswatches. I was going to move it to
the custom bootswatch but I'm not sure it's correct. It seems to be
making a link not look like a link, but I'm not sure why.

And the label tex should already inheret the correct color.
run grunt less over latest changes
Merge pull request #150 from ds125v/bootswatches_311

update bootswatches, add classic theme, reset
Merge pull request #151 from ds125v/environment_check

find-replace renderer for environment check table
Merge pull request #152 from ds125v/minor_tweaks

Minor tweaks
add icon translator, commented out