remove nav-tabs border rules from pagure.css
    
    Previously, we were using a rule in Pagure.css
    to remove the bottom border of nav-tabs elements.
    These nav-tabs were used in the repo-header tabs,
    the browsing tabs, and the preview tabs for comments.
    
    This commit removes this rule from the pagure.css, and
    replaces it in the places we need it in the html with the
    bootstrap utility class 'border-bottom-0'