UK: raise exception if unknown export_type is specified
UK: update docs
UK: test for api export endpoint
UK: use locally defined lookup_field
UK: allow use of id_string in export endpoint
UK: corrected test name to TestExportViewset, add xls export test that uses id_string
UK: remove url for ExportViewset, plan to merge ExportViewset functionality into XFormViewset
UK: allow use of id_string as pk for form access
UK: move data export to XFormViewset
UK: remove no longer required data export code