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

plugins/chat/spec/lib/statistics_spec.rb

@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+RSpec.describe Statistics do
+  describe "#participating_users" do
+    it "returns users who have sent a chat message" do
+      Fabricate(:chat_message)
+      expect(described_class.participating_users[:last_day]).to eq(1)
+    end
+
+    it "returns users who have reacted to a chat message" do
+      Fabricate(:chat_message_reaction)
+      expect(described_class.participating_users[:last_day]).to eq(2) # 2 because the chat message creator is also counted
+    end
+  end
+end