Add welcome chatmessages

* I added the option in config/config.exs
* created a new module lib/pleroma/user/welcome_chat_message.ex
* Added it to the registration flow
* added to the cheatsheet
* added to the config/description.ex
* added to the Changelog.md

test/user/welcome_chat_massage_test.exs

@@ -0,0 +1,35 @@
+# Pleroma: A lightweight social networking server
+# Copyright © 2017-2020 Pleroma Authors <https://pleroma.social/>
+# SPDX-License-Identifier: AGPL-3.0-only
+
+defmodule Pleroma.User.WelcomeChatMessageTest do
+  use Pleroma.DataCase
+
+  alias Pleroma.Config
+  alias Pleroma.User.WelcomeChatMessage
+
+  import Pleroma.Factory
+
+  setup do: clear_config([:welcome])
+
+  describe "post_message/1" do
+    test "send a chat welcome message" do
+      welcome_user = insert(:user)
+      user = insert(:user, name: "mewmew")
+
+      Config.put([:welcome, :chat_message, :enabled], true)
+      Config.put([:welcome, :chat_message, :sender_nickname], welcome_user.nickname)
+
+      Config.put(
+        [:welcome, :chat_message, :message],
+        "Hello. Welcome to blob.cat"
+      )
+
+      {:ok, %Pleroma.Activity{} = activity} = WelcomeChatMessage.post_message(user)
+
+      assert user.ap_id in activity.recipients
+      assert Pleroma.Object.normalize(activity).data["type"] == "ChatMessage"
+      assert Pleroma.Object.normalize(activity).data["content"] =~ "Hello. Welcome to "
+    end
+  end
+end