Adds peer review notification for assigning peer reviewers

Fixes: CNVS-3771

Adds a peer review notification to go out after
a peer reviewer is added to the discussion. This
also resolves the issue with a notification not
being sent if a rubric is not attached.

Test-Plan:
1.) Run bundle exec rake db:migrate in the console
	before doing anything else -- (This sets up your
	db by using notification_types.yml to populate
	notification types in the DB.)
2.) Create a graded discussion with at least 2
     students
3.) Make sure both students have everything
     in their notifications set to ASAP.
4.) Edit the assignment settings and select the
     'Require Peer Reviews' - Choose to manually
     select peer reviewers.
5.) Navigate to discussions as an admin
6.) Select the cog and go to peer reviews.
7.) Assign a peer reviewer and verify that the
     notification was sent out.

Change-Id: I78212cb8043de0fcf809a9ffa56ffca327ffcacc
Reviewed-on: https://gerrit.instructure.com/36646
Reviewed-by: Matthew Wheeler <mwheeler@instructure.com>
Product-Review: Matthew Wheeler <mwheeler@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>