feat: Add export_id field on Taxonomy

[ChrisChV]

Description

This PR adds a new export_id field on Taxonomy model.

Support information

• Github issue: https://github.com/openedx/modular-learning/issues/183
• Internal Ticket: FAL-3620

Testing instruction

• Verify that all tests pass and that they cover the requirements.

Before run migration

• Start a server with python manage.py runsever.
• Create a superuser with python manage.py createsuperuser.
• Go to Taxonomy list on Django Admin.
• Create two Taxonomies with the same name.
• Run the migrations: python manage.py migrate.
• Verify that the migrations have been executen without errors.
• On Django admin create a new Taxonomy and verify that you need to add an export_id
• Verify that you can edit the export_id on Django admin.
• Verify the validations of the export_id

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