add profile pics to mastery gradebook and outcome result api

fixes CNVS-10295

test plan
- regression on mastery gradebook
- ensure student profile pics appear next to the student name on
 the mastery gradebook when profile pics are enabled
- ensure that profile pics do not appear if profile pics are
 disabled

Change-Id: I24ccb21990402a76fe1c79ea39a752d1f73e8d5f
Reviewed-on: https://gerrit.instructure.com/28561
Reviewed-by: Braden Anderson <banderson@instructure.com>
QA-Review: Braden Anderson <banderson@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Joel Hough <joel@instructure.com>