update locked message on draft state discussions

fixes CNVS-8187

test plan:
  * in a course with draft state enabled, create a locked
    discussion (published w/ an available after date);
  * verify that the teacher view of the discussion does not
    change;
  * verify that as a student you see only the title of the
    discussion and a message that reads, "this topic is
    locked until <date>."

Change-Id: I8c7220752c337510b38802f37355c2583ba0287f
Reviewed-on: https://gerrit.instructure.com/28148
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Braden Anderson <banderson@instructure.com>
QA-Review: Matt Fairbourn <mfairbourn@instructure.com>
Product-Review: Zach Pendleton <zachp@instructure.com>