Fix #814: add timestamps to newsletters and waitlists in contact responses

[leplatrem]

I'm not a big fan for those del actual["create_timestamp"] in the tests.

We could either have a omit(dict, keys="") util, or, better IMO, we could have a test util to fuzzy match values instead of actual timestamp values.

For example:

https://github.com/mozilla/normandy/blob/8ec2b525493ee0af7da3e8beee4be4f1669f740c/normandy/base/tests/__init__.py#L12-L51

        assert serializer.data == {
            "signature": Whatever.regex(r"[a-f0-9]{40}"),
            "x5u": Whatever.startswith(signature.x5u),
            "timestamp": Whatever.iso8601(),
            "public_key": Whatever.regex(r"[a-zA-Z0-9/+]{160}"),
        }

https://github.com/mozilla/normandy/blob/8ec2b525493ee0af7da3e8beee4be4f1669f740c/normandy/recipes/tests/api/v1/test_serializers.py#L140-L145

[leplatrem]

@grahamalama r?