Allow "Description" and "Assigned To" to be prefilled in Maniphest's create task interface

Summary:
Ref T3320. Request from Dropbox. You can currently prefill "title" and some other stuff (including most of the fields by using templates) but there's no way to prefill description and assigned to right now.

  - Allow "Description" to be prefilled with a string ("description=...").
  - Allow "Assigned To" to be prefilled with a user rather than a PHID ("assign=username").

Test Plan:
Hit `/maniphest/task/create/?assign=epriestley&description=derpderp&title=blarpblarp` locally and got prefills.

{F45259}

Reviewers: chad, deuresti

Reviewed By: chad

CC: aran, euresti

Maniphest Tasks: T3320

Differential Revision: https://secure.phabricator.com/D6132