Render object remarkup references in a text context as "Dxxx <uri>"

Summary:
Depends on D19031. Fixes T11389. Currently, we render Dxxx in a text context (plain text email) as just a URI.

Instead, render it like Dxxx <uri>. This is more faithful to the original intent and preserves T123/T456 as two separate, usable links.

Test Plan: Wrote T123/T234 in a task, pulled mail for it with bin/mail show-outbound, saw separate clickable links.

Maniphest Tasks: T11389

Differential Revision: https://secure.phabricator.com/D19032