reST: Rendered .. sourcecode:: blocks have no top margin (BB-11101)

The <pre> elements generated from :: or .. code:: blocks get margin: 10px 0 0 0. .. sourcecode:: blocks are wrapped in a <div class="highlight"> and pre:first-child gets margin-top: 0, which has higher priority. <div> also gets margin: 0 and there is no special rule for .highlight. So there is no space between these blocks and the content above, which doesn't look very nice