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Add optional condition properties to reference questions:
- conditionalProperty: A property on the referenced form to be evaluated
- conditionalOperator: The operator that should be used for the comparison.
                       Supports all operators supported by Conditional Sections
- conditionalValue: The value that the property should be compared against
- conditionalFallback: The value that should be saved instead of the
                       referenced value if the condition is not satisfied
- conditionalType: The type of the property that is being compared

Refactored backend conditional section code in order to reuse conditional evaluation code

Update Reference Test forms to add testing support for new conditional references
- Added new section with 4 new questions to the bottom of ReferenceTestCopied.
  These questions are conditional on ReferenceTestUser being completed
- Made one question in ReferenceTestUser mandatory to allow the form to be edited
  between incomplete and complete

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Add optional error message when a conditional reference is not satisfied
- Allows non-text questions to have a text based error display

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- Add invalid source flag to reference questions with no source

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- Updated the import script to support conditional properties of reference questions

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- Clean up unused javadoc
- Add missing conditional reference columns to questionnaire import
- Add missing null checks