Allow Maniphest queries to have configurable page sizes

Summary:
Current page size is 1000. This is nice to have in some cases, but makes pages slower than necessary in others. Task lists are generally dominated by rendering costs.

For example, my default is "recent tasks", which just lists all tasks ordered by date created. Showing 100 tasks here instead of 1000 makes this several times faster without compromising utility.

I don't want to force the default to 100, though, since sometimes listing everything is quite useful and I think an advantage of Maniphest is that it generally deals reasonably well with large task sets.

(This limit property is actually read by the default implementation of getPageSize() in the parent class.)

Test Plan: Made queries with page sizes 1, 100, 12, 9, 3000, etc.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D6976