Fix an issue where some Differential edit pathways may not have reviewers attached

Summary:
Depends on D19021. Ref T13053. When you "Subscribe", or make some other types of edits, we don't necessarily have reviewer data, but may now need it to do the new recipient list logic.

I don't have a totally clean way to deal with this in the general case in mind, but just load it for now so that things don't fatal.

Test Plan: Subscribed to a revision with the "Subscribe" action.

Reviewers: amckinley

Maniphest Tasks: T13053

Differential Revision: https://secure.phabricator.com/D19022