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