Add an horizontal scrollbar on code snippets

Fixes #260

client/components/main/layouts.styl

@@ -317,6 +317,11 @@ a
     list-style-type: initial
     padding-left: 20px
 
+  pre
+    padding: 10px 12px 7px
+    background: darken(white, 13%)
+    overflow-y: auto
+
 .basicTabs-container .tabs-content-container
   padding: 0
   padding-top: 15px