Fix double-rendering of inline comments in mail

Summary: Ref T2222. This is a tmp.differential-only issue. Inline comment transactions now have content, so we treat them like body text. We also render them separately as inline text. This produces mail where inlines are rendered twice.

Test Plan: Sent myself mail, saw only one copy of inlines.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2222

Differential Revision: https://secure.phabricator.com/D8216