fix styling of input-group-addons
    
    WE previously used bootstrap input-group-addons to
    style up a few elements, specifically the git and ssh urls
    in the clone dropdown, and the listing of api keys in the
    settings for a project and a user.
    
    Since we were using pre-release versions of bootstrap 4 earlier,
    bootstrap changed slightly how these are marked up in newer versions.
    
    This commit changes them so they now are visually the same as
    with older versions.
    
    Fixes #3616