Use documentwrapper's height of sidebar (#9)

This is sort of a more correct thing to do since the first/last element of the body may have a margin that can cause the documentwrapper to be taller than bodywrapper.

While this doesn't cause any major issues for this theme as is (due to the ample use of whitespace around the sidebar), this behaviour would help with derivative themes which remove the excessive margins on the sidebar.

python_docs_theme/static/sidebar.js

@@ -28,6 +28,7 @@ $(function() {
   var jwindow = $(window);
   var jdocument = $(document);
   var bodywrapper = $('.bodywrapper');
+  var documentwrapper = $('.documentwrapper');
   var sidebar = $('.sphinxsidebar');
   var sidebarwrapper = $('.sphinxsidebarwrapper');
 
@@ -71,7 +72,7 @@ $(function() {
     bodywrapper.css('margin-left', bw_margin_collapsed);
     sidebarbutton.css({
         'margin-left': '0',
-        'height': bodywrapper.height(),
+        'height': documentwrapper.height(),
         'border-radius': '5px'
     });
     sidebarbutton.find('span').text('»');
@@ -85,7 +86,7 @@ $(function() {
     sidebarwrapper.show();
     sidebarbutton.css({
         'margin-left': ssb_width_expanded-12,
-        'height': Math.max(sidebarwrapper.height(), bodywrapper.height()),
+        'height': Math.max(sidebarwrapper.height(), documentwrapper.height()),
         'border-radius': '0 5px 5px 0'
     });
     sidebarbutton.find('span').text('«');
@@ -109,7 +110,7 @@ $(function() {
     // find the height of the viewport to center the '<<' in the page
     var viewport_height = get_viewport_height();
     var sidebar_offset = sidebar.offset().top;
-    var sidebar_height = Math.max(bodywrapper.height(), sidebar.height());
+    var sidebar_height = Math.max(documentwrapper.height(), sidebar.height());
     sidebarbutton.find('span').css({
         'display': 'block',
         'position': 'fixed',