Hightlighting the down-arrow next to user name on hover

- the right-arrow wasn't highlighted on hover, which was
  inconsistent with the other header items

- changed the user-menu items highlight color from #fff to #fafafa, which is
  consistent across the header

- refactored the css to reduce specificity

- refactored css to use nesting in class names instead of nesting in
  scss (as advocated by http://smacss.com) to reduce specificity and
  to enhance readability
Merge pull request #4441 from ssrihari/user-menu-highlight

Hightlighting the down-arrow next to user name on hover