add files from toolbar button

closes CNVS-11662

test plan:
  - enable new files
  - navigate to course files
  - select a specific Folder
    - click the '+ Files' button in top right
    - select a files (or a few)
      - files should be uploaded to the chosen Folder
      - once uploaded files should appear in the UI
        - refresh browswer to verify files are persisted

  caveats:
    - uploading progress UI will be handled in CNVS-12658
    - prompt for renaming files coming in CNVS-12667

Change-Id: I685d0336b1794f5bddd66cd21cc88d99b30ac662
Reviewed-on: https://gerrit.instructure.com/38793
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Ryan Shaw <ryan@instructure.com>
Product-Review: Jason Madsen <jmadsen@instructure.com>
QA-Review: Jason Madsen <jmadsen@instructure.com>