Moving a condition doesn't properly move the project objective conditions

After moving condition X from objective A to objective B, there are two project objective conditions (for each project):

- A hidden POC that links back to objective A and condition X.
- A displayed POC that links back to objective B and condition X.

The hidden POC is the one that existed before the move. It retains its completion status. But the completion status hasn't been copied across to the displayed POC.

The displayed POC should created by modifying the hidden POC, not by creating a fresh one.

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This bug also exposed a bug in the level calculation. The level calculation looks at all POCs that link back to condition X, which can make it impossible to achieve a level. We should separately improve the robustness of the level calculation by restricting the POC's objective.