Moved apps/devhub to mkt/developers (bug 1021197)

```
$ git grep for_apps
mkt/detail/views.py:    user_items = ActivityLog.objects.for_apps([addon]).exclude(
mkt/detail/views.py:    admin_items = ActivityLog.objects.for_apps([addon]).filter(
mkt/developers/models.py:    def for_apps(self, apps):
mkt/webapps/tests/test_tasks.py:        eq_(ActivityLog.objects.for_apps(self.addon).count(), 0)
mkt/webapps/tests/test_tasks.py:        eq_(ActivityLog.objects.for_apps(self.addon).count(), 1)
mkt/webapps/tests/test_tasks.py:        eq_(ActivityLog.objects.for_apps(self.addon).count(), 2)
mkt/webapps/tests/test_tasks.py:        eq_(ActivityLog.objects.for_apps(self.addon).count(), 2)
mkt/webapps/tests/test_tasks.py:        eq_(ActivityLog.objects.for_apps(self.addon).count(), 2)
mkt/webapps/tests/test_tasks.py:        eq_(ActivityLog.objects.for_apps(self.addon).count(), 2)
mkt/webapps/tests/test_tasks.py:        eq_(ActivityLog.objects.for_apps(self.addon).count(), 1)
mkt/webapps/tests/test_tasks.py:        eq_(ActivityLog.objects.for_apps(self.addon).count(), 2)
mkt/webapps/tests/test_tasks.py:        eq_(ActivityLog.objects.for_apps(self.addon).count(), 2)
```

The only place that passes the app model is tests. I could update the tests and remove this and make it accept a list.