Djangoevents

======

- Change the way modules are autodiscovered (use Django utility).
It should work now with new style apps.

======

- Added index on the `stored_entity_id` field which fixes performance
issue when there are many events. It requires migration, version
change reflects that.

======

- Switched to `schema_version = None` default when deserializing
events without `schema_version`.

======

- Stopped overriding `schema_version` if it exists.
- Started passing all event attributes to event constructor during
deserialization. This allows to handle event structure migration
("upcasting") in each event's constructor: one can modify `kwargs`
before calling `super().__init__`.

======

- Made `abstract` non inheritable: now only classes explicitly marked
with the `abstract` decorator are abstract. Their subclasses are not.
This was always the intended behavior.

======

- Switched to adding `schema_version` to event objects themselves
instead of just transcoded events. This way events will pass
validation if their Avro schemas require `schema_version`.

- Renamed setting responsible for the above feature
from `EVENT_TRANSCODER.ADDS_EVENT_VERSION_TO_DATA`
to `ADDS_SCHEMA_VERSION_TO_EVENT_DATA`.