Rewrite metrics stuff on top of new vumi metrics manager.
publish_oneshot_metrics -> publish_metrics
Django MetricPublisher implements appropriate interface.
Better get_published_metrics() helper.
Remove unused import.
Merge branch 'develop' into feature/issue-915-metrics-api
Merge branch 'develop' into feature/issue-915-metrics-api

Conflicts:
	go/vumitools/tests/test_app_worker.py
Merge branch 'feature/issue-932-better-VumiApi-publisher-setup' into feature/issue-915-metrics-api

Conflicts:
	go/vumitools/app_worker.py
Merge branch 'develop' into feature/issue-915-metrics-api
Move metric manager setup to VumiApi.
Switch MetricsMiddleware over to new-style metric manager.
Transient VumiApi for MetricsMiddleware.
Use default aggregators from Metric instead of specifying AGG.
Cleaner metric target construction.
Better construction of conversation metric targets.
Fix jsbox report view test.
Remove unused import.
Get default aggregator from Metric.
Merge branch 'feature/issue-915-metrics-api' into develop