feat: Add export_id field on Taxonomy

[ChrisChV]

Description

This PR adds a new export_id field on Taxonomy model.

Support information

• Github issue: [Tagging] An "Export ID" identifies each Taxonomy modular-learning#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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[ChrisChV]

This generated a warning on the lint

[pomegranited]

Couple of nits/questions, but everything else looks perfect. 👍

@ChrisChV Let me know if you need me to do a second review?

• I tested this in my devstack using the PR test instructions
• I read through the code
• I checked for accessibility issues N/A here
• Includes documentation -- adds help_text for new field, docstrings where needed
• User-facing strings are extracted for translation

[bradenmacdonald]

LGTM!

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