Generate feeds for spaces with Main.WebRss

Here is a list of improvements to generate feeds for spaces with the Main.WebRss script:

• the backward compatibility redirect does not take into consideration other URL parameters (e.g. if we have a space parameter, we lose it because http://localhost:8080/xwiki/bin/view/Main/WebRss?xpage=plain&space=Blog will redirect to http://localhost:8080/xwiki/bin/view/Main/WebRss?xpage=plain&outputSyntax=plain)
• generate a channel link considering the space parameter, otherwise use the Main space
• generate an empty feed, not an empty page if the user does not have view rights on the channel link (use $xwiki.getDocumentAsAuthor(...) instead of $xwiki.getDocument(...) for the $feedURI)