The menu arrow was previously implemented with a 45-degree rotated
square with border on two sides and white background. This had two
problems:

 - Because a square's diagonal is longer than its width, the right edge
   after rotation extended beyond the right edge of the content

 - The bottom part of the rotated square overlapped the menu content,
   which created a "notch" in the top menu item when hovered

Fix the problem and simplify the CSS by creating the triangle shape
using an SVG instead.