Allow markdown table to scroll (#4401)

* Allow markdown table to scroll

`overflow:auto` isn't effective when a table is wider than the container. Adding `display:block` fixes this issue.

public/less/_markdown.less

@@ -248,6 +248,7 @@
         overflow:auto;
         word-break:normal;
         word-break:keep-all;
+        display:block;
     }
 
     table th {