use hbox mixin instead of class

this will allow dynamic css in nbviewer like vbox-ing for low res
without JS or change in html

IPython/frontend/html/notebook/static/css/style.min.css

@@ -890,7 +890,7 b' div.ui-widget-content{border:1px solid #ababab;outline:none;}'
 .cell{border:1px solid transparent;}.cell.selected{border-radius:4px;border:thin #ababab solid;}
 div.cell{width:100%;padding:5px 5px 5px 0px;margin:2px 0px 2px 0px;outline:none;}
 div.prompt{width:11ex;padding:0.4em;margin:0px;font-family:monospace;text-align:right;line-height:1.231;}
-div.input{page-break-inside:avoid;}
+div.input{page-break-inside:avoid;display:-webkit-box;-webkit-box-orient:horizontal;-webkit-box-align:stretch;display:-moz-box;-moz-box-orient:horizontal;-moz-box-align:stretch;display:box;box-orient:horizontal;box-align:stretch;}
 div.input_area{border:1px solid #cfcfcf;border-radius:4px;background:#f7f7f7;}
 div.input_prompt{color:navy;border-top:1px solid transparent;}
 div.output_wrapper{margin-top:5px;margin-left:5px;width:100%;position:relative;}

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

@@ -114,7 +114,7 b' var IPython = (function (IPython) {'
 
         this.celltoolbar = new IPython.CellToolbar(this);
 
-        var input = $('<div></div>').addClass('input hbox');
+        var input = $('<div></div>').addClass('input');
         var vbox = $('<div/>').addClass('vbox box-flex1')
         input.append($('<div/>').addClass('prompt input_prompt'));
         vbox.append(this.celltoolbar.element);

IPython/frontend/html/notebook/static/less/notebook.less

@@ -207,6 +207,7 b' div.prompt {'
 
 div.input {
     page-break-inside: avoid;
+    .hbox();
 }
 
 /* input_area and input_prompt must match in top border and margin for alignment */