fix: "Copy Unit" button should be behind enable_copy_paste_units flag

[bradenmacdonald]

Description

This is a quick follow-up and fix for #33724 . The new Copy Unit button was appearing regardless of the contentstore.enable_copy_paste_units waffle flag. And once a unit was copied, the other new functionality could become visible.

This fixes the button so that it only appears when the contentstore.enable_copy_paste_units feature flag is explicitly enabled.

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Sandbox destroy request received.

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Sandbox destroy request received.

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[edx-pipeline-bot]

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

[edx-pipeline-bot]

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