Show open tasks on project pages

Summary: This is pretty basic but gets us most of the way there I think. Could
use some style tweaks at some point.
Test Plan: Looked at a project page with open tasks, and one without open tasks.
Reviewed By: tuomaspelkonen
Reviewers: cadamo, aran, jungejason, tuomaspelkonen
CC: aran, tuomaspelkonen, epriestley
Differential Revision: 609