Fixes broken link in peer review notification

Fixes: CNVS-15257

Changes the link in the peer review notification
so that it points to the right location when on
a different shard.

Test-Plan:
1.) Set up a graded discussion
2.) Add a peer reviewer
3.) Verify that a message gets sent to the reviewer
4.) Verify that the link points to the right location
4.5) Repeat steps on a different host and verify that	the link still points to the right place

Change-Id: Ib8c02a6d602234f7448306d829564572c0d6405d
Reviewed-on: https://gerrit.instructure.com/40580
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Alex Boyd <aboyd@instructure.com>
Reviewed-by: Brad Horrocks <bhorrocks@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Matt Fairbourn <mfairbourn@instructure.com>