Convert the default string datetime format returned by Elasticsearch to Unix timestamp format.

Summary:
Fixes T653.

This is intended as a quick fix so Edgehill is not blocked on it.
A better solution is to change our mapping and reconfigure the index,
this is what we'll do for prod.

Test Plan: Tested manually.

Reviewers: emfree

Reviewed By: emfree

Subscribers: bengotow

Maniphest Tasks: T653

Differential Revision: https://review.inboxapp.com/D859