Modernize MetaMTA message detail

Summary: Ref T3306. I'm going to add more information about To/Cc here, but here's a little cleanup first.

Test Plan: {F49524}

Reviewers: chad, btrahan

Reviewed By: chad

CC: aran

Maniphest Tasks: T3306

Differential Revision: https://secure.phabricator.com/D6410