ability to "add new folder" in new_files

closes: CNVS-12728

test plan:

* go to new files
* click the "+ Folder" button in the top
* it should create a new input in the list of files
  and auto-focus it.
* if you hit enter it should save
* if you hit escape while focused on that input
  it should cancel and do nothing

Change-Id: I2605f4d99560ed17ee3fb1a80350b83d4362a0f9
Reviewed-on: https://gerrit.instructure.com/39158
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jason Madsen <jmadsen@instructure.com>
Product-Review: Ryan Shaw <ryan@instructure.com>
QA-Review: Ryan Shaw <ryan@instructure.com>