## Bug description

When calculating Trends results with an aggregated (e.g. single output, like big number) display type, WAU/MAU don't work. This is because we have an `ACTIVE_USERS_SQL` query for time series display types, but we don't have a version of this for aggregated display types. (See ZEN-1043.)

## How to reproduce

1. Create an insight with a single WAU or MAU series.
2. Switch display type to "Number".
3. Open the "Debug ClickHouse queries" modal from the command palette.
4. See `COUNT(*)` instead of `COUNT(DISTINCT person_id)`.

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posthog/queries/test/test_trends.py
self._test_events_with_dates(
            dates=["2020-11-01", "2020-11-02", "2020-11-03", "2020-11-04"],
            interval="day",
            date_from="2020-11-01",
            date_to="2020-11-07",
            result=[
                {
                    "action": {
                        "id": "event_name",
                        "type": "events",
                        "order": None,
                        "name": "event_name",
                        "custom_name": None,