Append "Z" to DATETIME_FORMAT

We're formally using the ISO 8601 UTC datetime format, and so the
correct way to output the data is by appending the timezone.
("Z" in the case of UTC*).

Unfortunately, Python's `datetime` formatting will just ignore the
timezone part of the string on output, which means we just have to
append the string "Z" to the end of all datetime strings we output.

Should be fine, as we will only ever output UTC timestamps anyway.

* https://en.wikipedia.org/wiki/ISO_8601#UTC

app/__init__.py

@@ -18,7 +18,7 @@ from app.clients.statsd.statsd_client import StatsdClient
 from app.encryption import Encryption
 
 
-DATETIME_FORMAT = "%Y-%m-%dT%H:%M:%S.%f"
+DATETIME_FORMAT = "%Y-%m-%dT%H:%M:%S.%fZ"
 DATE_FORMAT = "%Y-%m-%d"
 
 db = SQLAlchemy()