## 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
# 
        _create_person(
            team_id=self.team.pk, distinct_ids=["blabla", "anonymous_id"], properties={"$some_prop": "some_val"}
        )
        _create_person(team_id=self.team.pk, distinct_ids=["third"])
        with freeze_time("2019-12-24 03:45:34"):
            _create_event(team=self.team, event="sign up", distinct_id="blabla")
            _create_event(
                team=self.team, event="sign up", distinct_id="blabla"
            )  # 
            _create_event(team=self.team, event="sign up", distinct_id="anonymous_id")
            _create_event(team=self.team, event="sign up", distinct_id="third")
        with override_instance_config("AGGREGATE_BY_DISTINCT_IDS_TEAMS", f"{self.team.pk},4"):
            with freeze_time("2019-12-31T13:00:01Z"):