Rss refresh error

This commit adds a warning on the RSS Refresh page and disables the refresh button if page content, which for RSS pages in the url, is missing.

It also provides a link back to the page so the user can edit the RSS feed and add the missing url.

In the best possible case the warning would be issued when the user tried to create a RSS page without the URL but it doesn't seem that comfortable mexican sofa provide this ability. One possibility would be to use a tag partial but there isn't any documentation on that feature so I'm not sure it's worth looking into.