FEATURE: participating users statistics (#28322)

Adds a new statistics (hidden from the UI, but available via the API) that tracks daily participating users.

A user is considered as "participating" if they have

- Reacted to a post
- Replied to a topic
- Created a new topic
- Created a new PM
- Sent a chat message
- Reacted to a chat message

Internal ref - t/131013

spec/lib/plugin/instance_spec.rb

@@ -105,6 +105,9 @@ TEXT
         :likes_7_days,
         :likes_30_days,
         :likes_count,
+        :participating_users_last_day,
+        :participating_users_7_days,
+        :participating_users_30_days,
       )
     end