Beginning of a responsive grid

Implement a responsive, fluid-ish grid.

This implementation is heavily based on Bootstrap's grid
(https://github.com/twbs/bootstrap/blob/master/less/grid.less and
https://github.com/twbs/bootstrap/blob/master/less/mixins.less),
the Golden Grid System (http://goldengridsystem.com/) and the
fluid 960 Grid System
(https://github.com/bauhouse/fluid960gs/blob/master/css/grid.css).

This is a good start to making the overall template responsive, but
there's still a lot of work to do, in particular reworking the navs
and header for the smaller sizes.

Addresses #774
Don't launch viewer in a modal popup

From the story detail page, don't launch the viewer in a modal
popup.  This is mostly because it breaks badly on mobile, but I've
never really liked this behavior anyway.

Addresses #774.
Responsive styles and markup for viewer

For smaller displays:

* Hide nav text
* Shrink logo width
* Stack two-up layouts

Addresses #774
Flatter next/previous buttons in viewer.

I always thought the gray background looked bad.
Get rid of header shell class, set breakpoints, fix indent

Addresses #774
Responsive header logo

Addresses #774
Hide top-nav menu on very narrow displays

Addresses #774
Make header search form responsive

On narrow displays, hide all the form elements except the submit
button.  This submits the form with an empty query, effectively
acting as a link to the search page.

Addresses #774
Merge branch 'develop' into 774-responsive_design

Cleanup of explorer CSS

Conflicts:
	static/css/style.css
Use a separate variable to toggle LESS in browser

For development, it makes sense to compile the LESS files in the
browser with less.js to make it easy to review changes.

I had been toggling this using ``settings.DEBUG``, but we also
need to be able to look at things in IE8, which isn't supported by
less.js.  Add a separate variable,
``settings.STORYBASE_USE_LESS_IN_BROWSER`` to toggle including the
LESS stylesheets (and less.js) in the template header.
Clean up header styles in preparation for responsive nav

* Fix indentation in LESS files
* Make full-width (``.container_12``) containers auto-width on
  narrow displays.
* Use IDs rather than classes for singular elements

Addresses #774
Move "My Account" menu into main megamenu

I think this will make responsiveness easier.  Also, I've always
hated how it kind of hung out there on the right.
Merge branch 'develop' into 774-responsive_design
Fix indentation, jshint cleanup
Remove unnecessary script tag type attribute
Factor megamenu jQuery interactions into a plugin

Do this in anticipation of putting responsive headers there too.

Addresses #774
First pass at toggling responsive menu.

Addresses #774
s/pluginMethods/methods/
Responsive footer

* Fix indentation and remove old styles from footer.less
* Rename classes for left and right columns so they don't conflict with
  the general .left and .right classes.
* Give the left and right columns grid_* classes to define width instead
  of hard coding it.

Addresses #774
Adjust order of footer columns

Adjust the order of the footer HTML elements so the nav will appear
above the copyright when the items are full-width on small displays.

Addresses #774