Backport PR #4159: don't split `.cell` and `div.cell` CSS

`.cell` matches non-IPython things that it shouldn't (inside highlighted code), everything `.cell` should match in IPython is a div.

I know we have bigger CSS scoping issues to deal with, but this is an easy fix for an obvious problem.

backport for 1.1

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

@@ -1,4 +1,4 b''
-.cell {
+div.cell {
     border: 1px solid transparent;
     .vbox();
 
@@ -6,9 +6,6 b''
         .corner-all;
         border : thin @border_color solid;
     }
-}
-
-div.cell {
     width: 100%;
     padding: 5px 5px 5px 0px;
     /* This acts as a spacer between cells, that is outside the border */

IPython/html/static/style/ipython.min.css

@@ -53,8 +53,7 b' input.engine_num_input{height:20px;margin-bottom:2px;padding-top:0;padding-botto'
 .ansibgpurple{background-color:magenta;}
 .ansibgcyan{background-color:cyan;}
 .ansibggray{background-color:gray;}
-.cell{border:1px solid transparent;display:-webkit-box;-webkit-box-orient:vertical;-webkit-box-align:stretch;display:-moz-box;-moz-box-orient:vertical;-moz-box-align:stretch;display:box;box-orient:vertical;box-align:stretch;width:100%;}.cell.selected{border-radius:4px;border:thin #ababab solid;}
-div.cell{width:100%;padding:5px 5px 5px 0px;margin:2px 0px 2px 7px;outline:none;}
+div.cell{border:1px solid transparent;display:-webkit-box;-webkit-box-orient:vertical;-webkit-box-align:stretch;display:-moz-box;-moz-box-orient:vertical;-moz-box-align:stretch;display:box;box-orient:vertical;box-align:stretch;width:100%;padding:5px 5px 5px 0px;margin:2px 0px 2px 7px;outline:none;}div.cell.selected{border-radius:4px;border:thin #ababab solid;}
 div.prompt{width:11ex;padding:0.4em;margin:0px;font-family:monospace;text-align:right;line-height:1.231em;}
 .celltoolbar{border:thin solid #CFCFCF;border-bottom:none;background:#EEE;border-top-right-radius:3px;border-top-left-radius:3px;width:100%;-webkit-box-pack:end;height:22px;}
 .no_input_radius{border-top-right-radius:0px;border-top-left-radius:0px;}

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

@@ -1434,8 +1434,7 b' input.engine_num_input{height:20px;margin-bottom:2px;padding-top:0;padding-botto'
 .ansibgpurple{background-color:magenta;}
 .ansibgcyan{background-color:cyan;}
 .ansibggray{background-color:gray;}
-.cell{border:1px solid transparent;display:-webkit-box;-webkit-box-orient:vertical;-webkit-box-align:stretch;display:-moz-box;-moz-box-orient:vertical;-moz-box-align:stretch;display:box;box-orient:vertical;box-align:stretch;width:100%;}.cell.selected{border-radius:4px;border:thin #ababab solid;}
-div.cell{width:100%;padding:5px 5px 5px 0px;margin:2px 0px 2px 7px;outline:none;}
+div.cell{border:1px solid transparent;display:-webkit-box;-webkit-box-orient:vertical;-webkit-box-align:stretch;display:-moz-box;-moz-box-orient:vertical;-moz-box-align:stretch;display:box;box-orient:vertical;box-align:stretch;width:100%;padding:5px 5px 5px 0px;margin:2px 0px 2px 7px;outline:none;}div.cell.selected{border-radius:4px;border:thin #ababab solid;}
 div.prompt{width:11ex;padding:0.4em;margin:0px;font-family:monospace;text-align:right;line-height:1.231em;}
 .celltoolbar{border:thin solid #CFCFCF;border-bottom:none;background:#EEE;border-top-right-radius:3px;border-top-left-radius:3px;width:100%;-webkit-box-pack:end;height:22px;}
 .no_input_radius{border-top-right-radius:0px;border-top-left-radius:0px;}