Fix the contest cloning bug

[magnified103]

Description

Type of change: bug fix

What

Fix the issue that problems are not properly copied to the destination contest when executing a contest cloning.

Why

The assignment of pk to None affects QuerySets that haven't been evaluated yet (not sure if these behaviors are intended). Using list() effectively forces the queries to perform actual db hits (link).

https://docs.djangoproject.com/en/4.2/topics/db/queries/#querysets-are-lazy

The performance impact of these changes is negligible for all practical use cases.

How Has This Been Tested?

Tested on local.

Checklist

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