Use active project filter as default field value when creating new tasks

Summary: See T389. If you're looking at tasks in project X, default new tasks
into project X.
Test Plan:

• Created a new task without associated projects.
• Created a new task from a filter view with 1 and 2 projects; defaults were

filled in.

• Submitted task with invalid title, projects were correctly preserved.

Reviewed By: skrul
Reviewers: skrul, hunterbridges, jungejason, tuomaspelkonen, aran
CC: aran, skrul, epriestley
Differential Revision: 757