feat!: add mango (maple) support for course creation assignment receiver

[mariajgrimaldi]

Description:
This PR adds support for our maple fork, since the issue BC-14 is not available in that version.

Previously, we'd implemented the automatic permissions assignment based on the customization BC-14 that was implemented in our lilac fork. Now, we're making it compatible with maple using the course creator model.

Installation instructions:

pip install -e git+https://github.com/eduNEXT/openedx-demo-plugin@MJG/mango-support#egg=openedx_demo_plugin==0.2.0

Testing instructions:

1. After installing the plugin, you must configure a staff users username to be used throughout the assignment:
   "COURSE_CREATOR_ADMIN_ID": "dummy-admin"
2. Create a user in the LMS
3. Check with staff permissions the LMS/CMS django admin, the following should exist:
   Course access role (LMS)
   
   Course creator role (CMS)
   
   
   Both org_content_creator_role & course creator are created since adding an organization to a current course creator creates a role in the access roles table.
4. Now, using the brand new user login to Studio. You must be able to create courses just in the OPEN_EDX_VISITOR_ORG

Reviewers:

• @felipemontoya
• @eduNEXT/service-delivery

Merge checklist:

• All reviewers approved
• CI build is green
• Version bumped
• Changelog record added
• Documentation updated (not only docstrings)
• Commits are squashed

Post merge:

• Create a tag
• Check new version is pushed to PyPI after tag-triggered build is
  finished.
• Delete working branch (if not needed anymore)

[mariajgrimaldi]

I'm currently implementing some tests ;)