Render Remarkup in emails

Summary: Ref T992. I noticed that ManiphestTask mail doesn't render Remarkup properly (instead, it renders Remarkup literally). I think this is because the code calls addTextSection() rather than addRemarkupSection().

Test Plan: Created a new Maniphest Task and saw Remarkup in the generated self-email (inspect the email contents with ./bin/mail show-outbound). I didn't test the other affected applications.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin

Maniphest Tasks: T992

Differential Revision: https://secure.phabricator.com/D14511