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/lib/chat/messages_exporter_spec.rb

@@ -0,0 +1,52 @@
+# frozen_string_literal: true
+
+describe Chat::MessagesExporter do
+  fab!(:public_channel) { Fabricate(:chat_channel) }
+  fab!(:public_channel_message_1) { Fabricate(:chat_message, chat_channel: public_channel) }
+  fab!(:public_channel_message_2) { Fabricate(:chat_message, chat_channel: public_channel) }
+  # this message is deleted in the before block:
+  fab!(:deleted_message) { Fabricate(:chat_message, chat_channel: public_channel) }
+
+  fab!(:private_channel) { Fabricate(:private_category_channel, group: Fabricate(:group)) }
+  fab!(:private_channel_message_1) { Fabricate(:chat_message, chat_channel: private_channel) }
+  fab!(:private_channel_message_2) { Fabricate(:chat_message, chat_channel: private_channel) }
+
+  fab!(:user_1) { Fabricate(:user) }
+  fab!(:user_2) { Fabricate(:user) }
+  fab!(:dm_channel) { Fabricate(:direct_message_channel, users: [user_1, user_2]) }
+  fab!(:direct_message_1) { Fabricate(:chat_message, chat_channel: private_channel, user: user_1) }
+  fab!(:direct_message_2) { Fabricate(:chat_message, chat_channel: private_channel, user: user_2) }
+
+  before { deleted_message.trash! }
+
+  it "exports messages" do
+    exporter = Class.new.extend(Chat::MessagesExporter)
+
+    result = exporter.chat_message_export.to_a
+
+    expect(result.length).to be(7)
+    assert_exported_message(result[0], public_channel_message_1)
+    assert_exported_message(result[1], public_channel_message_2)
+    assert_exported_message(result[2], deleted_message)
+    assert_exported_message(result[3], private_channel_message_1)
+    assert_exported_message(result[4], private_channel_message_2)
+    assert_exported_message(result[5], direct_message_1)
+    assert_exported_message(result[6], direct_message_2)
+  end
+
+  def assert_exported_message(data_row, message)
+    expect(data_row[0]).to eq(message.id)
+    expect(data_row[1]).to eq(message.chat_channel.id)
+    expect(data_row[2]).to eq(message.chat_channel.name)
+    expect(data_row[3]).to eq(message.user.id)
+    expect(data_row[4]).to eq(message.user.username)
+    expect(data_row[5]).to eq(message.message)
+    expect(data_row[6]).to eq(message.cooked)
+    expect(data_row[7]).to eq_time(message.created_at)
+    expect(data_row[8]).to eq_time(message.updated_at)
+    expect(data_row[9]).to eq_time(message.deleted_at)
+    expect(data_row[10]).to eq(message.in_reply_to_id)
+    expect(data_row[11]).to eq(message.last_editor.id)
+    expect(data_row[12]).to eq(message.last_editor.username)
+  end
+end