Backport PR #1113: Fix long lines escaping from the cell container

by setting:

    min-width: 0

on div.inner_cell ([reference](https://drafts.csswg.org/css-flexbox/#min-size-auto)).
...

notebook/static/notebook/less/cell.less

@@ -97,18 +97,11 @@ div.cell {
 }
 
 div.inner_cell {
+    min-width: 0;
     .vbox();
     .box-flex1();
 }
 
-@-moz-document url-prefix() {
-    div.inner_cell {
-        // hack around FF bug causing cell to expand when lines are long
-        // instead of scrolling
-        overflow-x: hidden;
-    }
-}
-
 /* input_area and input_prompt must match in top border and margin for alignment */
 div.input_area {
     border: 1px solid @light_border_color;