JU-6: add support for openedx-events

[mariajgrimaldi]

Description

This PR adds support for Open edX Events in Limonero. These events are part of the OEP-50: Hooks Extension Framework implementation plan.

Since the OEP-50 publication, there has been a lot of discussion surrounding the design of this new extension point; I'll address some here for context:

• These events are defined in the library openedx-events and will be imported here to be triggered in a specific list of places.
• The class OpenEdxPublicSignal -a subclass of Django signals- is used to create each event.
• send_event is the method that replaces send and send_robust.
• Each event sends payloads based on Data Attr classes. For more information: Open edX Events Payload Conventions
• Events unit test guarantees that they are being sent in the proper context. For example, STUDENT_REGISTRATION_COMPLETED is sent after the user's registration.

For an overall understanding of the design, check out the Open edX Events ADRs and discussions:

1. Open edX Event naming and versioning ADR
2. Open edX Event Payload Conventions ADR
3. Community discussion about Open edX Events design
4. Community discussion about Hooks Extension Framework

Supporting information

Discuss on Hooks Extension Framework:
https://discuss.openedx.org/t/configuration-for-the-hooks-extension-framework/4527/

Testing instructions

1. Checkout to li/ednx/JU-6_P2
2. Install Open edX Events library:
   pip install openedx-events==0.6.0
3. Connect your custom receiver to the signal as follows:

from openedx_events.learning.signals import STUDENT_REGISTRATION_COMPLETED

@receiver(STUDENT_REGISTRATION_COMPLETED)
def callback(**kwargs):

[MoisesGSalas]

@mariajgrimaldi How much does this differ from the upstream code? Where did you have to make adjustments if any?

[mariajgrimaldi]

Good question, @MoisesGSalas. Thanks.

1. In this platform version, the method _revoke_certificate does not exist, so we added CERTIFICATE_REVOKED to invalidate and mark_notpassing instead
2. The login reverse was not working, so I replaced it with this

The rest is the same:

1. Add event to process. i.e registration
2. Create test_events.py file for events related to the process
3. Add mixin which enable/disable events to test case classes that interact with the process. This is to avoid sending events when not needed.