Allow filtering maniphest tasks authored by agents

Summary:
Addes a button group to filter tasks by agents, non-agents or all.

Fixes T3394

Test Plan: View task list, filter by agents, filter by non agents. Make sure the correct tasks display.

Reviewers: epriestley, dctrwatson

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T3394

Differential Revision: https://secure.phabricator.com/D6328