Clean collection UDF values before showing them on the plot detail page

There was machinery in place to do this cleaning, but it requires retrieving
collection UDFs through the 'udf' property on UDFModel
Add support to udf_defs cache for getting all UDF defs for an instance
Clean audit values for UDFCollectionValue and scalar UDF audits

Also formats dates as part of Audit.current_display_value and
Audit.previous_display_value
Merge pull request #1609 from maurizi/fix-udf-date-formatting

Fix UDF date formatting