PERF: add index on user email logs

This index makes it much faster to figure out if a user reached
email limit on sites where lots of emails are skipped

db/migrate/20180207161422_add_skipped_created_at_user_id_index_on_email_logs.rb

@@ -0,0 +1,8 @@
+class AddSkippedCreatedAtUserIdIndexOnEmailLogs < ActiveRecord::Migration[5.1]
+  def up
+    execute "CREATE INDEX idx_email_logs_user_created_filtered ON email_logs(user_id, created_at) WHERE skipped = 'f'"
+  end
+  def down
+    execute "DROP INDEX idx_email_logs_user_created_filtered"
+  end
+end