Ensure dateCreated property exists at index creation time

Summary:
Since the default query will sort on this when no query string is
attached we want to make sure the property at least exists.

Otherwise Elasticsearch yells at you: "No mapping found for [dateCreated]
in order to sort on" when you try to search for documents that haven't
been indexed yet.

Test Plan:
Searched for Mocks and Initiatives (no such documents exist in my index)
and got the error. After patching and reinitializing the index, the error
during querying went away.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D11056