Inside the admin pages we don't have any navigation items inside the header. The empty navbar `<ul>` also did not have the proper class applied (`rb-c-topbar__nav-items`), which meant that we were getting an extra 12px of padding for an empty element. This change adds the class to the nav items element, and removes excess padding when that element is empty. While in here I also fixed some broken border rules on the header `__action` element (`ink-g-header-border` was perhaps supposed to be `ink-g-border-header`, but we don't want a border on these at all) as well as some bad indentation. Testing Done: Verified that the header inside the admin pages no longer had unexplained extra padding.