fixes CNVS-65, CNVS-3921
test plan:
* copy app/messages/new_discussion_entry.email.erb to
/app/messages/new_discussion_entry.email.html.erb;
* create a new discussion in a class with at least two
users (who have their discussion entry notifications
set to "immediately");
* as user 1, post a reply to the discussion;
* verify that user 2 receives a multipart email with
an html part and a plaintext part.
Change-Id: I30e3e15fa53817767c770491ab8c19286281e0aa
Reviewed-on: https://gerrit.instructure.com/17964
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jon Willesen <jonw@instructure.com>
QA-Review: Marc LeGendre <marc@instructure.com>
Zach Pendleton