## 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._create_groups()
        journey = {
            "person1": [
                {
                    "event": "sign up",
                    "timestamp": datetime(2020, 1, 2, 12),
                    "properties": {"$group_0": "org:5"},
                    "group0_properties": {"industry": "finance"},
                },
                {
                    "event": "sign up",
                    "timestamp": datetime(2020, 1, 2, 13),
                    "properties": {"$group_0": "org:6"},
                    "group0_properties": {"industry": "technology"},
                },
                {
                    "event": "sign up",
                    "timestamp": datetime(2020, 1, 2, 15),
                    "properties": {"$group_0": "org:7", "$group_1": "company:10"},
                    "group0_properties": {"industry": "finance"},
                    "group1_properties": {"industry": "finance"},
                },
            ]
        }
        journeys_for(events_by_person=journey, team=self.team)
        filter = Filter(
            data={