Transfer of border-box-sizing from js to css

`border-box-sizing` is pure styling, it has nothing to do in Javascript
it should be put in css to be overwritable.

IPython/html/static/notebook/js/codecell.js

@@ -123,7 +123,7 b' define(['
     CodeCell.prototype.create_element = function () {
         Cell.prototype.create_element.apply(this, arguments);
 
-        var cell =  $('<div></div>').addClass('cell border-box-sizing code_cell');
+        var cell =  $('<div></div>').addClass('cell code_cell');
         cell.attr('tabindex','2');
 
         var input = $('<div></div>').addClass('input');

IPython/html/static/notebook/js/textcell.js

@@ -70,7 +70,7 b' define(['
     TextCell.prototype.create_element = function () {
         Cell.prototype.create_element.apply(this, arguments);
 
-        var cell = $("<div>").addClass('cell text_cell border-box-sizing');
+        var cell = $("<div>").addClass('cell text_cell');
         cell.attr('tabindex','2');
 
         var prompt = $('<div/>').addClass('prompt input_prompt');

IPython/html/static/notebook/less/cell.less

@@ -2,19 +2,18 b' div.cell {'
     border: 1px solid transparent;
     .vbox();
     .corner-all();
+    .border-box-sizing();
     border-width: thin;
     border-style: solid;
-    
+
     &.selected {
         border-color: @border_color;
     }
-    
+
     &.edit_mode {
         border-color: green;
     }
-}
 
-div.cell {
     width: 100%;
     padding: 5px 5px 5px 0px;
     /* This acts as a spacer between cells, that is outside the border */