refactor: change default page size to 15

okay so it seems sorting then ordering by `ended`
is really messing up the DB

we need an index with tags + batch_ended, let's
see if that is viable...

but for now, with the current optimizations and paging
to 15 EG is already pretty faster

Author: arcstur

editgroups/settings/common.py

@@ -110,7 +110,7 @@
         'rest_framework.renderers.JSONRenderer',
      ),
     'DEFAULT_PAGINATION_CLASS': 'store.pagination.PaginationWithoutCounts',
-    'PAGE_SIZE': 50,
+    'PAGE_SIZE': 15,
     'DEFAULT_AUTHENTICATION_CLASSES': (
         'rest_framework.authentication.BasicAuthentication',
         'rest_framework.authentication.SessionAuthentication',