Return task descriptions from "maniphest.search"

Summary:
Fixes T12461. This returns the field as a dictionary with a "raw" value, so we could eventually do this if we want without breaking the API:

{
  "type": "remarkup",
  "raw": "**raw**",
  "html": "<strong>raw</strong>",
  "text": "raw"
}

Test Plan: Called maniphest.search, reviewed output.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12461

Differential Revision: https://secure.phabricator.com/D17603