R1:57761ce2202a

Differential - re-jigger mail such that inline comments show up right after the main comments.

Summary: Ref T6962. Mainly accomplished by re-factoring the base editor `buildMailBody` function and then using it differently in the `DifferentialTransactionEditor`.

Test Plan: commented on a revision leaving inline feedback. inspected via bin/mail and it looked good! also made a maniphest comment and checked that email, which still looked good.

Reviewers: chad, epriestley

Reviewed By: epriestley…