Summary:
Ref T2852. Two issues:

  - Embeds (`T12`, `{T12}`) have some handle issues because handles run afoul of visibility checks under some configs. Make handles unconditionally visible.
  - Asana links don't render correctly into text mode. Give them a valid text mode rendering so they don't flip out.

Test Plan: Made comments with `T12` and `http://app.asana.com/...` and published them to Asana.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2852

Differential Revision: https://secure.phabricator.com/D6696