make codemirror use visible tabs

* this may not be optimal (compared to e.g. replacing tabs with spaces), but it's a lot better than mystery 'bad indentation' errors

common/lib/xmodule/xmodule/css/capa/display.scss

@@ -416,6 +416,12 @@ section.problem {
     line-height: 18px;
     resize: both;
 
+    .cm-tab {
+        background: url(data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAADAAAAAMCAYAAAAkuj5RAAAAAXNSR0IArs4c6QAAAGFJREFUSMft1LsRQFAQheHPowAKoACx3IgEKtaEHujDjORSgWTH/ZOdnZOcM/sgk/kFFWY0qV8foQwS4MKBCS3qR6ixBJvElOobYAtivseIE120FaowJPN75GMu8j/LfMwNjh4HUpwg4LUAAAAASUVORK5CYII=);
+        background-position: right;
+        background-repeat: no-repeat;
+    }
+    
     pre {
       @include border-radius(0);
       border-radius: 0;