Fix counting impressions based using distinct play_id instead of page_view_id

[matus-tomlein]

Describe the bug

Impressions in the media base table are counted as the number of distinct page views. However, this is imprecise as there may be multiple videos per page, which then leads to the play rate being higher than 1 (the number of plays is more than the number of page views).

Expected results

A better solution would be to use the number of distinct play_id as the number of impressions. This would take into account all the loaded videos on the page.