TinyMCE: Made the Insert Image plugin treeview accessible

Fixes CNVS-5670

Test plan:
- On any RCE, click the button to insert an image.
- Select the Canvas tab.
- Using the keyboard, verify that the treeview is navigable via the arrow
  keys, enter, home and end.
- Confirm that the space bar is functionally the same as the enter key.
- Verify that the treeview is likewise accessible and navigable when using a
  screen reader.

Change-Id: Ib45d9f56dc6e7a7168ba13c59296db59504e69a4
Reviewed-on: https://gerrit.instructure.com/28317
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Myller de Araujo <myller@instructure.com>
Reviewed-by: Derek DeVries <ddevries@instructure.com>
Product-Review: Aaron Cannon <acannon@instructure.com>