Explicitly list To and Cc in multiplexed email

Summary:
When we multiplex email, add information to the body with an explicit list of
recipients. Also add some headers if people want to write mail rules.

Test Plan:
Commented on a task and a revision, got reasonable looking emails about them.

Reviewed By: jungejason
Reviewers: aran, jungejason, tuomaspelkonen
CC: aran, jungejason, epriestley
Differential Revision: 272