removed link to users on activity stream conversations
removed link to users on activity stream conversations

fixes CNVS-3337

test steps:
- go to the dashboard after receiving conversation messages
- ensure that the sender names aren't linked

Change-Id: I6803bcbffb2b97c604d7048d6533d9768db64e8c
Reviewed-on: https://gerrit.instructure.com/19460
Reviewed-by: Mark Ericksen <776ab05fc6b68e82d2b2c63873bf56edde131809@instructure.com>
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
QA-Review: Marc LeGendre <d33c80bc45d65303e33ca83108a9952b745af9ef@instructure.com>
Product-Review: Eric Berry <b4e131942074b4e4dafdcc14a63e85b7d8e18577@instructure.com>

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