Explicitly list reviewers on reviewrequest email

Summary: This used to be in the subject but there was a bunch of churn and now
it's nowhere.
Test Plan: Created, updated, and added CCs to a diff.
Reviewed By: moskov
Reviewers: moskov, avitaloliver, jungejason, tuomaspelkonen, aran
CC: aran, moskov
Differential Revision: 567