Re-add file duration constraint and re-enable tests

[bjester]

Summary

Description of the change(s) you made

• Re-adds the file duration constraint, dropped in Temporarily disable file duration check constraint #4041, since it does allow null values when the preset is one of MEDIA_PRESETS
• Django drops and re-adds the constraint so the net result is no different

References

Fixes #4040
Prior PR #4041

Comments

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[vkWeb]

@bjester looks good sir! 💯