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/requests/site_controller_spec.rb

@@ -68,6 +68,8 @@ RSpec.describe SiteController do
       expect(json["likes_count"]).to be_present
       expect(json["likes_7_days"]).to be_present
       expect(json["likes_30_days"]).to be_present
+      expect(json["participating_users_7_days"]).to be_present
+      expect(json["participating_users_30_days"]).to be_present
     end
 
     it "is not visible if site setting share_anonymized_statistics is disabled" do