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

lib/pleroma/user/welcome_chat_message.ex

@@ -0,0 +1,45 @@
+# Pleroma: A lightweight social networking server
+# Copyright © 2017-2020 Pleroma Authors <https://pleroma.social/>
+# SPDX-License-Identifier: AGPL-3.0-only
+
+defmodule Pleroma.User.WelcomeChatMessage do
+  alias Pleroma.Config
+  alias Pleroma.User
+  alias Pleroma.Web.CommonAPI
+
+  @spec enabled?() :: boolean()
+  def enabled?, do: Config.get([:welcome, :chat_message, :enabled], false)
+
+  @spec post_message(User.t()) :: {:ok, Pleroma.Activity.t() | nil}
+  def post_message(user) do
+    [:welcome, :chat_message, :sender_nickname]
+    |> Config.get(nil)
+    |> fetch_sender()
+    |> do_post(user, welcome_message())
+  end
+
+  defp do_post(%User{} = sender, recipient, message)
+       when is_binary(message) do
+    CommonAPI.post_chat_message(
+      sender,
+      recipient,
+      message
+    )
+  end
+
+  defp do_post(_sender, _recipient, _message), do: {:ok, nil}
+
+  defp fetch_sender(nickname) when is_binary(nickname) do
+    with %User{local: true} = user <- User.get_cached_by_nickname(nickname) do
+      user
+    else
+      _ -> nil
+    end
+  end
+
+  defp fetch_sender(_), do: nil
+
+  defp welcome_message do
+    Config.get([:welcome, :chat_message, :message], nil)
+  end
+end