[Admin] Reduce server charge on dashboard

[HugoGresse]

The dashboard need to get all the votes and then make the aggregation in the client.
We should either store those result OR process it from the backend and update in only every 15min when the page is fetched. So when the dashboard needs data, we get it and then if the last refresh is more than 15min ago, we run a cloud function to update it.
We could probably do that in live too.

I think we should wait for the completion of #46 before engaging this.