Improvement/pdp import data and button to export subjects table to excel

Added migration to support extra field in worker to write comments. Added button to export subjects table to excel Closes #291 (closed) #290 (closed)

smash/web/api_views/subject.py

@@ -294,7 +294,10 @@ def subjects(request, type):
 
         filtered_subjects = get_subjects_filtered(all_subjects, filters)
         ordered_subjects = get_subjects_order(filtered_subjects, order_column, order_dir, column_filters=dict(filters))
-        sliced_subjects = ordered_subjects[start:(start + length)]
+        if length == -1:
+            sliced_subjects = ordered_subjects
+        else:
+            sliced_subjects = ordered_subjects[start:(start + length)]
 
         result_subjects = sliced_subjects