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/discourse_hub_spec.rb

@@ -62,6 +62,8 @@ RSpec.describe DiscourseHub 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
         expect(json["installed_version"]).to be_present
         expect(json["branch"]).to be_present
       end