Add a call to predict the next event for a trigger
Summary: Ref T6881. This is useful to show a "Next backup: 2:30 AM" sort of thing without requring callers to know how triggers work internally.
Test Plan: Showed that kind of thing in Instances.
Reviewers: btrahan
Reviewed By: btrahan
Subscribers: epriestley
Maniphest Tasks: T6881
Differential Revision: https://secure.phabricator.com/D11437