When there are over 5M records on `action` table, the heatmap on
dashboard is very slow as below SQL.
```
database duration=1.8881s db.sql="SELECT created_unix DIV 900 * 900 AS timestamp, count(user_id) as contributions FROM `action` WHERE user_id=? AND act_user_id=? AND (created_unix > ?) GROUP BY timestamp ORDER BY timestamp"
```
This PR add a new index for `action` table with columns `user_id`,
`act_user_id` and `created_unix` so that this query will become about 6
times faster than before.
4a7ab0abf0