so that event-routing-backends can be dependency for non-edx-platform
repos like openedx-completion-aggregator.

* Pulls the EVENT_TRACKING_BACKENDS configuration template into a
  utility method so it can be used by other apps.
* Pulls allowed xAPI and Caliper events into settings to preserve
  changes
* Adds a test

…ings

instead of global django.conf.settings

in class variable name and in a comment.

Refactors the transformer test classes to split the test functionality into two:

* TransformersFixturesTestMixin -- for running the event fixture tests
* TransformersTestMixin -- for running the ERB-specific event tests

The fixtures file path constants have been moved to property methods so
they can be overrideen when used outside of ERB.

This exception may occur when using ERB transformer test mixins because
the fixture test files are not installed with this package.

…tings

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

This allows other plugins to modify these settings too.

and fix coverage