feat: Add tags count to taxonomy serializer [FC-0036]

[ChrisChV]

Testing instructions

• You can test this change with feat: Taxonomy delete dialog [FC-0036] frontend-app-authoring#684 (comment)
• Install this branch on the cms
• Follow the testing instructions in feat: Taxonomy delete dialog [FC-0036] frontend-app-authoring#684 (comment)
• Verify that the tags count is showed in the delete dialog

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[bradenmacdonald]

@ChrisChV Please add a test that verifies this field is included in the REST API response(s)

[ChrisChV]

Please add a test that verifies this field is included in the REST API response(s)

@bradenmacdonald Sure. Added here 503bd0d

[bradenmacdonald]

👍

• I tested this: confirmed the count is in the API response
• I read through the code
• I checked for accessibility issues: n/a
• Includes documentation: n/a

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