fix all-around sidebar heading borders
    
    Previously, on some of the sidebar headers on issue-related
    pages had borders all around the heading, rather than just on
    the bottom.
    
    This fixes this for the related prs heading on issues page, and
    the metadata heading on the new issue page.
    
    Fixes: #3744
    
    Signed-off-by: Ryan Lerch <rlerch@redhat.com>