Tweak notification styles
    
    Adds all notifications for a single request into a single
    box that can be more easily dismissed when there are multiple
    notifications. Also absolutely positions the notification so we
    don't have to worry about if a page uses a sub-header or not (like
    on the repo pages)
    
    The notification body is now white, and the status of each notification
    is signifed by the text colour and a small icon.
    
    The welcome message still uses the alert-info colours.
    
    Relates to #3347
    
    Signed-off-by: Ryan Lerch <rlerch@redhat.com>