Herald - add support for "assignee" conditions

Summary: add support for "assignee" conditions

Test Plan: Create a Herald rule where condition is assignee, and create a task assign to someone.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D7813