fix user roster page scrolling

test plan:
* add more than 50 people to a course

 the easiest way to do this would probably be an sis import,
 or running the following in the console:

 c = Course.find(INSERT_COURSE_ID_HERE)
 60.times { c.enroll_user(User.create!) }

* confirm that the people page scrolls and
 paginates correctly

fixes #CNVS-13646

Change-Id: If84251963e5502dddb5fbdac54c98b9d64bfa128
Reviewed-on: https://gerrit.instructure.com/36790
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>