When mail (like "!history" mail) has multiple comments, label them separately

Summary:
Depends on D19372. Ref T13124. See PHI505. Currently, if you !history a task with a lot of comments, you get output like this:

alice added a comment.
bailey added a comment.
alice added a comment.
alice added a comment.

AAAA

BBBB

AAAA

AAAA

This is impossible to read. Put the "alice added a comment." headers above the actual comments for comments after the first.

These types of mail messages are unusual, but occur in several cases:

• The new !history command.
• Multiple comments on a draft revision before it promotes out of draft.
• (Probably?) Conduit API updates which submit multiple comment transactions for some reason.

Test Plan: Used bin/mail receive-test to send a !history command to a task, saw a much more readable rendering of the transaction log in the resulting email.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13124

Differential Revision: https://secure.phabricator.com/D19373