R1:10c6fcb4bbf3

Allow Maniphest filtering by "upforgrabs" explicitly

Summary:
Allow you to filter by "upforgrabs" to find unassigned tasks.

Test Plan:
Filtered by "upforgrabs".

Reviewed By: tuomaspelkonen
Reviewers: tuomaspelkonen, jungejason, aran
Commenters: aran
CC: sandra, anjali, aran, tuomaspelkonen, epriestley
Differential Revision: 365