Enable prefilling of some Maniphest fields in task creation

Summary:
This came up in discussions with both ccheever and fratrik so I prototyped a
"send screenshot to maniphest" feature, which needs this:

https://www.facebook.com/video/video.php?v=892599296749

Test Plan:
Sent screenshot to maniphest.

Reviewed By: aran
Reviewers: jungejason, tuomaspelkonen, aran
CC: ccheever, fratrik, aran, epriestley
Differential Revision: 240