Bug 1075645 - Metrics dashboard

I like the idea of offline processing of the metrics. To make it more efficient you can:

* add `Task.last_attempt_timestamp` 
* add a django_signal to listen for saves and deletes of TaskAttempts and update the last_attempt_timestamp field
* In this loop instead of `all()` filter only the Tasks with timestamp >= last_cronjob_run

This way you can scale a lot more, but maybe that's an exercise for the future :)