Heading hierarchy is inconsistent in aggregator pages

The same problem happens on aggregator/sources and aggregator/categories: they are structured as:

<h1>(site name)</h1>
<h2>Sources | Categories</h2>
<h2>Source 1 | Category 1</h2>
(content: sources | categories)
....
<h2>Source 2 | Category 2</h2>
...

whereas Source|Category number "n" are semantically part of the general Sources|Categories, and should therefore be included in a h3, not a h2. The attached patch fixes this.