conversations: fix auto-linking of www. in messages

fixes CNVS-2630

test plan:
  * open conversations
  * send a message containing a URL with no protocol
      starting with www.
  * confirm that the URL becomes a link which works correctly

Change-Id: I028e4eb386eed3307d10fc60884f0de008f2601c
Reviewed-on: https://gerrit.instructure.com/36444
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Drew Bowman <dbowman@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Braden Anderson <braden@instructure.com>