Switch to Atom output, so we can include atom:summary

[Athanasius]

Once the output is using fulltext in the <description> element it may be useful to consumers to have an <atom:summary> element containing the old precis text.

Note that the W3C validator tells you off for using <atom:content>, advising instead to just stick to <description> so that doesn't need to change.

Unfortunately the perl module XML::RSS currently used for the feed output doesn't have very good support for atom elements. We can put an <atom:summary> element in, but can't set an attribute type="html" which is necessary because we're embedding encoded HTML in it. If nothing else there's the 'see more' <a href="..."/> we put in.

So this would entail finding a different perl module for Atom output.