FEATURE: Show draft count in user menu and activity (#13812)

This commit adds the number of drafts a user has next to the "Draft"
label in the user preferences menu and activity tab. The count is
updated via MessageBus when a draft is created or destroyed.

db/migrate/20210720221817_add_draft_count_to_user_stat.rb

@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+class AddDraftCountToUserStat < ActiveRecord::Migration[6.1]
+  def change
+    add_column :user_stats, :draft_count, :integer, default: 0, null: false
+
+    execute <<~SQL
+      UPDATE user_stats
+      SET draft_count = new_user_stats.draft_count
+      FROM (SELECT user_stats.user_id, COUNT(drafts.id) draft_count
+            FROM user_stats
+            LEFT JOIN drafts ON user_stats.user_id = drafts.user_id
+            GROUP BY user_stats.user_id) new_user_stats
+      WHERE user_stats.user_id = new_user_stats.user_id
+        AND user_stats.draft_count <> new_user_stats.draft_count
+    SQL
+  end
+end