Fix loadtestdata not working after adding a new model

[david-venhoff]

Short description

This pr attempts to fix the problem that the test data cannot be loaded anymore after a new model gets added.

I apologize for the huge diff :see_no_evil: is there maybe a way to reduce the diff, other than doing it manually?

Proposed changes

• Regenerate the test data with ./tools/integreat-cms-cli dumpdata --natural-primary --natural-foreign --o test_data.json
• Handle an exception that occurs when loading feedback and the base object is not available yet

Resolved issues

Fixes: #2763

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