Un-hide the "X added reviewers: ..." transactions in revision creation mail

Summary:
Fixes T12118. See PHI54. This adds a special case for the initial "reviewers" transactions, similar to the existing special case for "projects" transactions.

Although these transactions are redudnant in the web view since you can see the information clearly on the page, they're more reasonably useful in mail.

Test Plan:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12118

Differential Revision: https://secure.phabricator.com/D18542