Stretch goal: If appropriate, keep some fieldMappings after file reload

[choilmto]

After loading a PDF, then loading a CSV, and mapping at least one CSV column to one PDF field, the fieldMappings array is no longer empty. If you then reload a PDF, retain the mappings for PDF fields that are in both the previously loaded PDF file and the newly loaded PDF file. Meanwhile, when reloading a CSV, retain the mappings for CSV column headers that are in both the previously loaded CSV file and the newly loaded CSV file.

In the case where the user does not want to retain mappings after creating a batch of PDFs, there needs to be a reset button to clear fieldMappings.