Add CSV export mode and system worker

goes with arkindex/workers/export!20 (merged)

arkindex/process/models.py

@@ -71,6 +71,7 @@ class ExportFormat(Enum):
     PDF = "pdf"
     PageXML = "page_xml"
     DOCX = "docx"
+    CSV = "csv"
 
 
 class Process(IndexableModel):
@@ -705,6 +706,7 @@ class ArkindexFeature(Enum):
     ExportPDF = "pdf_export"
     ExportPageXML = "pagexml_export"
     ExportDOCX = "docx_export"
+    ExportCSV = "csv_export"
 
 
 class WorkerVersion(models.Model):
@@ -1225,5 +1227,6 @@ class CorpusWorkerVersion(models.Model):
 FEATURE_FORMAT_MAP = {
     ExportFormat.PDF: ArkindexFeature.ExportPDF,
     ExportFormat.PageXML: ArkindexFeature.ExportPageXML,
-    ExportFormat.DOCX: ArkindexFeature.ExportDOCX
+    ExportFormat.DOCX: ArkindexFeature.ExportDOCX,
+    ExportFormat.CSV: ArkindexFeature.ExportCSV
 }