Stream item api supports Assessment Requests

Fixes CNVS-16147

We simply neglected to support assessment requests
in the stream item api when we added assessment
requests to the stream items.

Test Plan:
1)  As an instructor, create a course with at least
	two students and an assignment, with peer reviews enabled.
	(Enable peer reviews under the assignment edit screen.)
2)  From the assignment show page, click 'peer reviews'
	in the right-hand nav under "Related Items"
3)  Assign a peer review for student A to student B.
4)  Hit the stream item api endpoint for student B
	and verify the data looks correct and the api
	doesn't error out. To do this, masquerade as that
	user, then in your browser hit
	`http://localhost:3000/api/v1/users/self/activity_stream.json`

	response should look like this:
	   while(1);
	  [{"created_at":"2014-10-10T21:09:30Z",
	  "updated_at":"2014-10-10T21:09:31Z",
	  "id":11,"title":"Peer Review for Bar Homework",
	  "message":null,
	  "type":"AssessmentRequest",
	  "read_state":true,
	  "context_type":"Course",
	  "course_id":1,
	  "html_url":"http://localhost:3000/courses/1/assignments/2/submissions/3"}]

Change-Id: Id097ca434783ffa741b4fad1faaf38087a1c384a
Reviewed-on: https://gerrit.instructure.com/42681
Reviewed-by: Joel Hough <joel@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Matthew Wheeler <mwheeler@instructure.com>