added last activity to roster page

test plan:

1. go to courses/:id/users as somebody who can
   manage students or admins in the course (like
   a teacher)

   - observe the new column with the last activity

2. go the same page as a user who can't manage
   students in the course (like a student)

   - observe the column is not there

Change-Id: I85a84142fd740b5d61e236fabda8a8e8fdbd82ad
Reviewed-on: https://gerrit.instructure.com/31799
QA-Review: Matt Fairbourn <mfairbourn@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Mark Severson <markse@instructure.com>
Product-Review: Hilary Scharton <hilary@instructure.com>