Add task to regenerate ballot_lines_count cache

We've added this column recently to `budget_ballots`, and we use it to
calculate stats.

lib/tasks/regenerate_ballot_lines_cache.rake

@@ -0,0 +1,8 @@
+namespace :budgets do
+  desc "Regenerate ballot_lines_count cache"
+  task calculate_ballot_lines: :environment do
+    Budget::Ballot.find_each do |ballot|
+      Budget::Ballot.reset_counters ballot.id, :lines
+    end
+  end
+end