Redirect cover letter message-id URLs from /patch/msgid/<message-id> to the series

[jnikula]

http://patchwork.freedesktop.org/patch/msgid/<message-id> for patches redirects to http://patchwork.freedesktop.org/patch/<patch-id>.

It would be useful to have http://patchwork.freedesktop.org/patch/msgid/<message-id> for cover letters to redirect to the appropriate http://patchwork.freedesktop.org/series/<series-id>.