calendar2: limit context selection to 10 items

fixes CNVS-2221

test plan:
  * create more than ten courses
  * open calendar2
  * try to select more than ten contexts
  * verify that your selection is limited to ten items

Change-Id: I17c5f87a7929433613c2cb25bc7b89d858353fd2
Reviewed-on: https://gerrit.instructure.com/33266
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Zach Pendleton <zachp@instructure.com>
Product-Review: Zach Pendleton <zachp@instructure.com>
QA-Review: Zach Pendleton <zachp@instructure.com>