## 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/trends/sql.py
VOLUME_SQL = """
SELECT {aggregate_operation} as data, {interval}(toTimeZone(toDateTime(timestamp, 'UTC'), %(timezone)s) {start_of_week_fix}) as date FROM ({event_query}) GROUP BY date
"""
VOLUME_AGGREGATE_SQL = """
SELECT {aggregate_operation} as data FROM ({event_query}) events
"""
VOLUME_PER_ACTOR_SQL = """
SELECT {aggregate_operation} AS data, date FROM (
    SELECT COUNT(*) AS intermediate_count, {aggregator}, {interval}(toTimeZone(toDateTime(timestamp, 'UTC'), %(timezone)s) {start_of_week_fix}) AS date
    FROM ({event_query})