[FC-0036] fix: allows taxonomy list to "filter by org" even if org does not exist

[pomegranited]

If the user requests the taxonomies linked to a non-existent org, then we want to see the non-org taxonomies.

Description

Allows the "taxonomy list" endpoint to filter by org even if the requested org is not found in the Organizations list.
Previously, attempting to filter taxonomies by a non-existent org would throw an error (400 Bad Request).
With this change, filtering by an non-existent org will return only the (global) instance-level taxonomies.

This case comes up in devstack (the edX Organization for the DemoX is not automatically created), and may also affect other small deployments which don't use Organizations.

Supporting information

Came across this while testing PRs for openedx/modular-learning#118

Testing instructions

1. If you don't already have sample taxonomy/tags data, follow the instructions in this repo to generate sample data: https://github.com/open-craft/taxonomy-sample-data

2. Visit the "taxonomy list" endpoint, with no filters: http://localhost:18010/api/content_tagging/v1/taxonomies/

   All taxonomies should be returned.

3. Visit the "taxonomy list" endpoint and filter by a non-existent org, e.g. http://localhost:18010/api/content_tagging/v1/taxonomies/?org=edX

   Only those taxonomies with all_orgs=True should be listed.

Deadline

None

Other information

Private-ref: FAL-3523

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[yusuf-musleh]

@pomegranited Thanks for the fix! It looks great to me! 👍

• I tested this: (followed the testing instructions)
• I read through the code
• I checked for accessibility issues
• Includes documentation
• I made sure any change in configuration variables is reflected in the corresponding client's configuration-secure repository.

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2U Release Notice: This PR has been deployed to the edX staging environment in preparation for a release to production.

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2U Release Notice: This PR has been deployed to the edX production environment.