avatar image fills circle

fixes CNVS-12018

test plan
- view a non-square avatar
- ensure that the bordering circle is filled

Change-Id: I70d635b4ac908294c75173d8474a852781f0966e
Reviewed-on: https://gerrit.instructure.com/32976
Reviewed-by: Braden Anderson <banderson@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Joel Hough <joel@instructure.com>