Always save level after cloning concept difficulty

[bethanyaconnor]

The level concept difficulty was cloning correctly in the database but wasn't being written to the level file. This is because we write the file in an after_save hook, but we were only saving when changed? was true. However, when a relationship is assigned, the object itself isn't changed, so changed? returns false.

I couldn't figure out a good way to update the unit tests for this but I did confirm locally that this change writes the level concept difficulties on clone.

PR Checklist:

• Tests provide adequate coverage
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