add custom query logic for aggregate_all_ever relationship

[bcodell]

Resolves #36

This PR:

• adds custom query logic in the dataset macro to more efficiently compute aggregations for aggregate_all_ever relationship. It does so by bypassing the join to the primary activity, which is feasible since the aggregated columns have no time dependency on the primary activity. It instead aggregates around the customer column for the joined activity, and then joins back to the primary activity CTE's customer column in the final CTE that joins all the CTEs together.
• updates docs to warn developers against specifying additional join logic that uses the {{ primary() }} macro in aggregate_all_ever activities

Generated sql for dataset__aggregate_all_ever_1:

with filter_activity_stream_using_primary_activity as (
    select
        stream.activity_id as activity_id,
        stream.entity_uuid as entity_uuid,
        stream.ts as ts,
        stream.revenue_impact as revenue_impact,
        stream.activity as activity,
        stream.activity_occurrence as activity_occurrence,
        stream.activity_repeated_at as activity_repeated_at

    from "dbt"."main"."input__aggregate_all_ever" as stream
    where stream.activity = 'signed up'
        and (true)
),
append_and_aggregate__1__aggregate_all_ever
 as (
    select
    -- special case for aggregate_all_ever relationship to avoid exploding join
        entity_uuid,
        count(appended.activity_id) as aggregate_all_ever_visit_page_activity_id
    from "dbt"."main"."input__aggregate_all_ever" as appended
    where appended.activity = 'visit page'
    group by entity_uuid

),
rejoin_aggregated_activities as (
    select
        stream.activity_id,
        stream.entity_uuid,
        stream.ts,
        stream.revenue_impact,
    append_and_aggregate__1__aggregate_all_ever.aggregate_all_ever_visit_page_activity_id
    from filter_activity_stream_using_primary_activity as stream
    left join append_and_aggregate__1__aggregate_all_ever
        on append_and_aggregate__1__aggregate_all_ever.entity_uuid = stream.entity_uuid
)

select * from rejoin_aggregated_activities