Update _top-bar.scss

Fix to three issues:

Removing the background from .top-bar-section ul (line 286) prevents bleed through of dropdown background colour onto the top level if ul elements have had padding declared elsewhere

Adding padding: 0; to .dropdown (line 385) ensures that dropdown menus align the the top menu item and sit alongside each other whether or not ul elements have had padding declared elsewhere

Changes to line heights (lines 484, 491, and 549)  make top level links the same height as the Top Bar removing the "overhang" at the bottom, while keeping the line height for dropdown links
Merge pull request #4946 from clyates/patch-1

Update _top-bar.scss