Fix API fields filtering

[Guflly]

Proposed Changes

Use the requested fields list when rendering user and course API responses instead of always returning every default field.

Closes #8071

Screenshots of your changes (if applicable)

Not applicable.

Type of Change

Type	Applies?
Bug fix	X

Checklist

Before opening your pull request:

• I have performed a self-review of my changes.
• I have added tests for my changes, if applicable.
• I have updated the project documentation, if applicable.
• If this is my first contribution, I have added myself to the list of contributors.

After opening your pull request:

• I have updated the project Changelog.
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• I have reviewed the test coverage changes reported by Coveralls.
• I have requested a review from a project maintainer.

Questions and Comments

Local checks: RuboCop on the changed Ruby files and Ruby syntax checks. The controller specs are included for CI.