FEATURE: Export chat messages to CSV file (#22113)

To export chat messages, go to `/admin/plugins/chat` and click the Create export 
button in the _Export chat messages_ section. You'll receive a direct message 
when the export is finished.

Currently, this exports all messages from the last 6 months, but not more than 
10000 messages.

This exports all chat messages, including messages from private channels and 
users' direct conversations. This also exports messages that were deleted.

plugins/chat/spec/requests/admin/export_controller_spec.rb

@@ -0,0 +1,40 @@
+# frozen_string_literal: true
+
+require "rails_helper"
+
+RSpec.describe Chat::ChatController do
+  describe "#export_messages" do
+    fab!(:user) { Fabricate(:user) }
+    fab!(:moderator) { Fabricate(:moderator) }
+    fab!(:admin) { Fabricate(:admin) }
+
+    it "enqueues the export job and logs into staff actions" do
+      sign_in(admin)
+
+      post "/chat/admin/export/messages"
+
+      expect(response.status).to eq(204)
+
+      expect(Jobs::ExportCsvFile.jobs.size).to eq(1)
+      job_data = Jobs::ExportCsvFile.jobs.first["args"].first
+      expect(job_data["entity"]).to eq("chat_message")
+      expect(job_data["user_id"]).to eq(admin.id)
+
+      staff_log_entry = UserHistory.last
+      expect(staff_log_entry.acting_user_id).to eq(admin.id)
+      expect(staff_log_entry.subject).to eq("chat_message")
+    end
+
+    it "regular users don't have access" do
+      sign_in(user)
+      post "/chat/admin/export/messages"
+      expect(response.status).to eq(403)
+    end
+
+    it "moderators don't have access" do
+      sign_in(moderator)
+      post "/chat/admin/export/messages"
+      expect(response.status).to eq(403)
+    end
+  end
+end