Adds the arrow to the top of the navigation menu

This is the last big piece for [#51593657]: the top border includes a sort of arrow up to the menu button to visually indicate the link between itself and the control which opened it. (Added two empty divs to the HTML which got styled to represent the "arrow".)

https://www.pivotaltracker.com/story/show/51593657
Adds up and down arrow glyphs to HAS themes

This adds the down white arrow to the "MENU" trigger, and the up grey arrows to the activities inside the menu. There are notes on the last glyph, the white down arrow next to the active activity, which is a little trickier so far.

[#51593657]

https://www.pivotaltracker.com/story/show/51593657