speed up the popularity/movers update by running bulk queries (bug 1059964)

Fixes [bug 1059964](https://bugzilla.mozilla.org/show_bug.cgi?id=1059964)

The idea is to do a "bulk update" using a temporary table:
1/ compute (two queries) the average number of users over a week and over three weeks
2/ bulk insert the popularity and movers computed from those numbers in the temporary table
3/ We can then
```sql
UPDATE personas, tmp_table
SET personas.popularity = tmp_table.popularity,
    personas.movers = tmp_table.movers
WHERE personas.id = tmp.personas_id```
What if the persona has been deleted between stats collecting and script run?