Fix CSS path prefix when S3 storage is used

[lo48576]

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• [x] Bug Fix
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• [ ] Plumbing / Internals / Dependencies
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Description

django-sass-processor 1.4 looks up OPTIONS using sass_processor instead of staticfiles. Add sass_processor field to STORAGES config, in order to let django-sass-processor 1.4 look up the OPTIONS. This change will fix CSS path to have /static prefix.

• Closes #3383

Documentation

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