In preparation for introducing batched e-mails, this splits up each of

our e-mail templates into containers (which just have headers like the

auto-generated email blob and footers like signatures) and content.

There's a tiny bit of clean-up done but no actual changes have been made

to the content of the emails themselves.

Looked at email preview views for review requests, reviews, and review

replies in both html and text formats. Saw everything work as expected.