profile pics: display round border on cropping interface

fixes CNVS-12382

test plan:
  * open your profile page
  * edit your avatar
  * select an image file
  * verify that the cropping interface presents a round border

Change-Id: I23842540c9923916d95031495c5f609fd897fad6
Reviewed-on: https://gerrit.instructure.com/33260
Reviewed-by: Joel Hough <joel@instructure.com>
Product-Review: Braden Anderson <banderson@instructure.com>
QA-Review: Braden Anderson <banderson@instructure.com>
Tested-by: Braden Anderson <banderson@instructure.com>