Use batches of 100

This is much better from a performance point of view - batches of 500
take a long time (55sec) but we can run batches of 100 in under 60ms.

app/services/issue_rebalancing_service.rb

@@ -18,7 +18,7 @@ class IssueRebalancingService
     start = RelativePositioning::START_POSITION - (gaps / 2) * gap_size
 
     Issue.transaction do
-      indexed_ids.each_slice(500) { |pairs| assign_positions(start, pairs) }
+      indexed_ids.each_slice(100) { |pairs| assign_positions(start, pairs) }
     end
   end