Use nanoscroller to allow intro doc scrolling

app/styles/play/level/loading.sass

@@ -89,8 +89,7 @@ $UNVEIL_TIME: 1.2s
     .intro-doc
       text-align: left
       font-size: 16px
-      //overflow-y: scroll  // bad scroll bars
-      overflow: hidden
+      //overflow-y: scroll  // using nanoscroller now
 
       img
         max-width: 100%

app/templates/play/level/level_loading.jade

@@ -10,7 +10,10 @@
 
   .errors
 
-  .intro-doc
+  .intro-doc.hidden
+    .nano
+      .nano-content
+        .intro-doc-content
 
   .progress-or-start-container
     button.start-level-button.btn.btn-lg.btn-success.btn-illustrated.header-font.needsclick(data-i18n="play_level.loading_start") Start Level

app/views/play/level/LevelLoadingView.coffee

@@ -155,8 +155,10 @@ module.exports = class LevelLoadingView extends CocoView
     minHeight = $('#code-area').outerHeight(true)
     @$el.css height: maxHeight
     @$loadingDetails.css minHeight: minHeight, maxHeight: maxHeight
-    $intro = @$el.find('.intro-doc')
-    $intro.css maxHeight: minHeight - $intro.offset().top - @$el.find('.progress-or-start-container').outerHeight() - 30 - 20
+    if @intro
+      $intro = @$el.find('.intro-doc')
+      $intro.css height: minHeight - $intro.offset().top - @$el.find('.progress-or-start-container').outerHeight() - 30 - 20
+      $intro.find('.nano').nanoScroller alwaysVisible: true
 
   unveilWings: (duration) ->
     @playSound 'loading-view-unveil', 0.5
@@ -167,7 +169,7 @@ module.exports = class LevelLoadingView extends CocoView
   unveilIntro: =>
     return if @destroyed or not @intro or @unveiled
     html = marked utils.filterMarkdownCodeLanguages(utils.i18n(@intro, 'body'))
-    @$el.find('.intro-doc').html html
+    @$el.find('.intro-doc').removeClass('hidden').find('.intro-doc-content').html html
     @resize()
 
   onUnveilEnded: =>