Rewrite issue RSS generator as an action

Currently to generate an issue's RSS, we have a whole bunch of if-then logic in issuedetails-rss.jsp iterating through fields and objects of an issue, and rendering them. All this logic should be written as code in an action, which would construct a DOM object of some sort, and then pass this to the JSP for rendering as text.

This should also eliminate encoding bugs (JRA-3398), as an XML library presumably does a better job ensuring generated text is valid than we do.