Adjusts handling of the folder browser

Makes it so that the "active" item is highlighted/bolded depending on the situation.
It also makes it so that keyboard navigation has a focus effect.

fixes CNVS-15596

Test Plan:
  - Enable Better File Browsing
  - Go to New Files
  - Navigate folders using the left side navigation
  - The actively displayed folder should be bold.
  - When you use the keyboard to go up and down, a blue background should be there.
  - Move a file using the Move dialog
  - The currently selected folder should have a blue background.

Change-Id: I260750b41a41d39f925b33e8b84c5b0984d0d7a2
Reviewed-on: https://gerrit.instructure.com/43249
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Ryan Shaw <ryan@instructure.com>
Product-Review: Ryan Shaw <ryan@instructure.com>
QA-Review: Jahnavi Yetukuri <jyetukuri@instructure.com>