Allow prefilling a task's assignee by his PHID

Summary: Some scripts might find it easier to work with PHIDs instead of user names.

Test Plan:
Use ?assign=<username> and ?assign=<PHID-USER> with the create task URI.
See assignee input being filled correctly.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: epriestley

CC: epriestley, aran, Korvin

Differential Revision: https://secure.phabricator.com/D7401