doesn't reset collaborator picker list when loading more

fixes CNVS-8264

test plan
- have a course with more than 50 users (more than 100 is better)
- navigate to the "start a new collaboration" page
- scroll to the bottom of the "collaborate with" user picker
- the users should load in groups of about 50 when you approach
 the bottom of the list
- ensure that when all users are loaded and you've reached the
 bottom of the list, you can scroll back to the top and no users
 have been removed from the list

Change-Id: Iff29f880375872afd6a48bf606e93f39236707c8
Reviewed-on: https://gerrit.instructure.com/30130
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Braden Anderson <banderson@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Joel Hough <joel@instructure.com>