[Fix bug 997275] Build event metrics management interface.

* Remove old metrics related_name.
* Add EventMetric and EventMetricOutcome models.
* Show EventMetric to base content.
* Fix /stats/ URL conflict with admin URL.
* Register new models to admin.
* Minor fixes in new models Meta.
* Replace old metrics with new metrics.
* Remove unused code from old metrics.
* Change templates to handle new metrics.
* Minor compatibility fixes between old and new metrics.
* Add factory for new metrics