Allow longer values for pdf_key

If a pdf_key is too long, this error will happen when uploading the PDF:

String data, right truncated: 1406 Data too long for column 'pdf_key' at row 1: INSERT INTO {fillpdf_fields} (fid, pdf_key, label, value)

Processing stops on a pdf_key that is too long and the remaining fields are not inserted resulting in being able to only fill some of the fields.

To make matters worse, it still gives the "was successfully uploaded" message, so it is not clear that there was a problem with the upload.

There at least needs to be a warning about the problem. Perhaps the length limit should be removed.