Minor NavBar fixes
NavBar: Hide scrollbars in firefox and chrome
Add LESS mixins for flexbox

Browsers implement flexbox in several different ways (different attribute
names and values). This change adds LESS mixins to abstract away the
differences in flexbox syntax across browsers. This change also:
  - Adds unit tests for the mixins
  - Modifies the grunt tasks to compile LESS files in the tests directory
Fix #6: Switch NavBar from grid to flexbox

NavBar uses CSS grid which only works in IE. Specifically:
  - In NavBarCommand, CSS grid was used to ensure that each command's
    label shrinks by the right amount when icons and/or split buttons are
    enabled.
  - In NavBar, CSS grid was used to position the NavBarCommands.

Both of these uses of CSS grid were switched to flexbox to achieve
identical layouts.