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

config/config.exs

@@ -261,6 +261,11 @@ config :pleroma, :welcome,
     sender_nickname: nil,
     message: nil
   ],
+  chat_message: [
+    enabled: false,
+    sender_nickname: nil,
+    message: nil
+  ],
   email: [
     enabled: false,
     sender: nil,