R1:2047483cc046

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

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