After invoking expand/collapse button in navigation pane, keyboard focus is lost.

[agniuks]

Found during accessibility grading.

Tracking internal bug: https://microsoft.visualstudio.com/DefaultCollection/OS/_workitems/edit/40136021

[agniuks]

components on the page "behind" the open nav pane remain in the tab order

[agniuks]

closely tied to #222, and would both get fixed by addressing the react-navigation issue. We could force the focus to the expand/collapse button when clicking on it, but that would not fix the fact that components of the previous page remain in the tab order.