Allow Maniphest to scale to a massive size

Summary:
Maniphest is missing some keys and some query strategy which will make it
cumbersome to manage more than a few tens of thousands of tasks.

Test Plan:
Handily manipulated 100k-scale task groups. Maniphest takes about 250ms to
select and render pages of 1,000 tasks and has no problem paging and filtering
them, etc. We should be good to scale to multiple millions of tasks with these
changes.

Reviewed By: gc3
Reviewers: fratrik, jungejason, aran, tuomaspelkonen, gc3
Commenters: jungejason
CC: anjali, aran, epriestley, gc3, jungejason
Differential Revision: 534