Fix rights typing in OpenAPI docs

ReDoc was displaying the corpus rights array as "rights": [null] (an array of anything) because of an easy mistake with ListField: ListField(CharField()) is perfectly valid, but it does not mean what one might think it means, since it would try to serialize the CharField itself; the correct value is ListField(child=CharField()).

assigned to @erwanrouchet

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requested review from @babadie

added 4 commits

• a27a1629...f25a523b - 3 commits from branch master
• e5868fd5 - Fix rights typing in OpenAPI docs

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added 5 commits

• e5868fd5...b6f8c4fa - 4 commits from branch master
• c5de6cb3 - Fix rights typing in OpenAPI docs

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enabled an automatic merge when the pipeline for c5de6cb3 succeeds

mentioned in commit 647476f0

merged