We had code in the review request page that checked for the presence of

the Review.status_update attribute. This in turn triggered a query for

the object on the other end of that relation, regardless of whether it

existed. That led to a useless query per review, especially since we had

already fetched all status updates on the page.
This change adds some pre-processing, creating a map of reviews to

status updates, and utilizes that map instead of performing the

attribute check.
This offers a pretty large savings on most review requests.

Unit tests passed.
Verified in Django Debug Toolbar that the repeated SQL queries for

status updates were no longer present.