add keyboard navigation to content select area

fixes CNVS-10401

Summary: This adds the proper key bindins so you can navigate the
select content area with your keyboard.

Test Plan 1 (keyboard navigation)
1. Import content into a course or do a course copy where you are
required to select content.
2. Once the select content area pops up, hit tab twice so you are now
focused on the content inside of the select content area.
3. You should see the first time highlighted in blue. This means that
item is currently selected.
4. Use the keyboards up and down arrows to navigat the tree. Use the
left and right arrows to expand/collapse items.
5. Use the space bar to select and deselect times.

Test Plan 2 (accessiblity)
1. Do everything in test plan 1 except use accessiblity tools to do it.

Change-Id: I6febb23d6709e290434d82472ce5020ad77d1679
Reviewed-on: https://gerrit.instructure.com/29913
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Aaron Cannon <acannon@instructure.com>
Reviewed-by: Aaron Cannon <acannon@instructure.com>
QA-Review: Matt Fairbourn <mfairbourn@instructure.com>