Recognize Maniphest task description as a remarkup block

Summary:
Ref T4589. We don't recognize task descriptions as remarkup blocks, so `{F...}` references in them do not get attached to the objects, and thus no policy exemption is created.

Recognize them, which activates `{F...}` and `@mentions`.

We probably have a few more of these in other applications, but it's not a big deal to clean them up as they arise.

Test Plan: Uploaded a file to a task in the description field, saw it attach and get a policy exemption.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4589

Differential Revision: https://secure.phabricator.com/D10139