PERF: Cache user badge count in user_stats table (#8610)

This means that we no longer need to run a `count()` on the user_badges table when serializing a user

db/migrate/20191220134101_add_distinct_badge_count_to_user_stats.rb

@@ -0,0 +1,19 @@
+# frozen_string_literal: true
+class AddDistinctBadgeCountToUserStats < ActiveRecord::Migration[6.0]
+  def change
+    add_column :user_stats, :distinct_badge_count, :integer, default: 0, null: false
+
+    execute <<~SQL
+      UPDATE user_stats
+      SET distinct_badge_count = x.distinct_badge_count
+      FROM (
+        SELECT users.id user_id, COUNT(distinct user_badges.badge_id) distinct_badge_count
+        FROM users
+        LEFT JOIN user_badges ON user_badges.user_id = users.id
+                              AND (user_badges.badge_id IN (SELECT id FROM badges WHERE enabled))
+        GROUP BY users.id
+      ) x
+      WHERE user_stats.user_id = x.user_id AND user_stats.distinct_badge_count <> x.distinct_badge_count
+    SQL
+  end
+end