Put cows and figlet bannners in <pre> in HTML mail bodies

Summary: Fixes T9538. Ref T9408. cowsay and figlet Remarkup rules are being mangled in HTML mail right now. Put them in <pre> to unmangle them.

Test Plan:
Sent myself a cow + figlet in mail.

Used bin/mail show-outbound --id ... --dump-html > dump.html + open that HTML file in Safari to preview HTML mail.

Saw linebreaks and monospaced formatting.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9538, T9408

Differential Revision: https://secure.phabricator.com/D14248