Add rake task to calculate cached voted score

Run this task it's only necessary if already existing legislation proposals in DB.

lib/tasks/legislation_proposals.rake

@@ -0,0 +1,10 @@
+namespace :legislation_proposals do
+  desc "Calculate cached votes score for existing legislation proposals"
+  task calculate_cached_votes_score: :environment do
+    Legislation::Proposal.find_each do |p|
+      p.update_column(:cached_votes_score, p.cached_votes_up - p.cached_votes_down)
+      print "."
+    end
+    puts "\nTask finished 🎉"
+  end
+end