Enable prefilling of projects and priority fields in Maniphest task creation

Summary:
Projects and priority inputs can be prefilled similar to how title
and description fields work.

Prefilling of projects already worked but used PHIDs instead of
more user friendly name so I changed that too.

Test Plan:
Visit [[/maniphest/task/create/?projects=Maniphest;Easy&priority=100&assign=vrana&title=Hip-hip&description=hooray!|example]]
and see prefilled form fields.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D7394