genericizing file/folder tree to refer to treeItems

fixes CNVS-9047

test plan:
- enable better file browsing feature flag
- go to files page and navigate folders, all expected behaviors should occur
- click 'move folder' and navigate the folder tree, all expected behaviors should occur,
including files moving properly
- go to edit addignment, edit outcome, or edit discusson and click 'embed image' on toolbar
- go to 'Canvas' tab on modal window and navigate tree, all expected behaviors should occur,
including images properly attaching

Change-Id: I703f0db5a096d49d648fab8377001b5fd610a620
Reviewed-on: https://gerrit.instructure.com/43883
Reviewed-by: Simon Williams <simon@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Nathan Rogowski <nathan@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>