For MPP-1639: Add RelayAddress.used_on field

[groovecoder]

This PR is for MPP-1639.

How to test:

1. python manage.py migrate
2. http://127.0.0.1:8000/api/v1/relayaddresses/
   • You should see a new used_on field
3. Use the django admin site to add some comma-separated domains into the new used_on field of an address. (e.g., disneyplus.com,netflix.com
4. Go to http://127.0.0.1:8000/api/v1/relayaddresses/?used_on=disney
   • You should see the alias matching "disney" in the list!
5. Go to http://127.0.0.1:8000/api/v1/relayaddresses/?used_on=hulu
   • You should NOT see the alias matching "disney" in the list!

• I've added a unit test to test for potential regressions of this bug.
• Commits in this PR are minimal and have descriptive commit messages.

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[groovecoder]

Note: I updated this PR to add django-filter library so we can filter API list views on ANY of the model fields by adding a filterset_class to any ModelViewSet.

E.g.:

• http://127.0.0.1:8000/api/v1/relayaddresses/?used_on=disney (contains match)
• http://127.0.0.1:8000/api/v1/relayaddresses/?generated_for=disneyplus.com (exact match)
• http://127.0.0.1:8000/api/v1/relayaddresses/?enabled=true (exact match)

django-filter can allow any kind of url parameter matching described in the Django "Field lookups" docs - exact, partial/contains, greater/less-than (numbers or dates), etc.

So, are there other API endpoints we could add some of the url param lookups to simplify code on the front-end?

[jwhitlock]

Thanks @groovecoder, the tests steps worked. I've got one question and one field type suggestion - see in-line comments.

[groovecoder]

Changed to TextField. The field is written by the add-on, so it can't be in read_only_fields.

[jwhitlock]

Thanks @groovecoder! 👏 good job re-writing the migration! (watch out @maxxcrawford if you migrated)