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