respect 'include' parameter in /groups/x/users API

when fetching the users in a group via the
/api/v1/groups/:group_id/users endpoint, respect the same set of
include[] query parameter flags as when listing users in a course.

refs MBL-1961

test-plan:
 - have a group with a user with a specified avatar
 - fetch /api/v1/groups/<that group>/users
 - returned user record should not have an avatar_url field (default)
 - fetch /api/v1/groups/<that group>/users?include[]=avatar_url
 - returned user record should have an avatar_url field

Change-Id: Ib7eda3b077faa79e2a8a1a4c8481775c1d54aa0e
Reviewed-on: https://gerrit.instructure.com/32962
Reviewed-by: Rob Orton <rob@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Jacob Fugal <jacob@instructure.com>