use branch from openedx/event-routing-backends#431

* adds dependency on edx-event-routing-backends
* adds transformers to xapi.completion and xapi.progress
* adds the completion_aggregator events to the event tracking whitelist

and adds CHANGELOG entry.

test settings and plugin settings.

The previous commit resulted in the
COMPLETION_AGGREGATOR_ASYNC_AGGREGATION flag being flipped for some
tests -- it was True in test_settings, but False in plugin_settings()

So for these tests, we're now overriding that setting as needed.

…_completion_events

and rearrange INSTALLED_APPS to fix tests.

Uses transforms fixture test utilities from ERB to ensure that raw
aggregator events are transformed to xAPI as expected.

* test_output/ -- used by ERB to write failed event transforms for debugging
* adds 'factory' as a test requirement because ERB tests needs it.

…_completion_events

not Score.scaled

Ordinary completion events are simply "progress" events with a recorded
progress %, so we should do the same here -- no need for "completed"
events.

…_completion_events

…tings

Initialize EVENT_TRACKING_BACKENDS_ALLOWED_XAPI_EVENTS and
EVENT_TRACKING_BACKENDS_ALLOWED_CALIPER_EVENTS if they aren't
already, and append our events to them.

ERB will do the same to preserve our settings if they're loaded first.