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,

config/description.exs

@@ -997,6 +997,35 @@ config :pleroma, :config_description, [
           }
         ]
       },
+      %{
+        group: :chat_message,
+        type: :group,
+        descpiption: "Chat message settings",
+        children: [
+          %{
+            key: :enabled,
+            type: :boolean,
+            description: "Enables sends chat message for new user after registration"
+          },
+          %{
+            key: :message,
+            type: :string,
+            description:
+              "A message that will be sent to a newly registered users as a chat message",
+            suggestions: [
+              "Hi, @username! Welcome on board!"
+            ]
+          },
+          %{
+            key: :sender_nickname,
+            type: :string,
+            description: "The nickname of the local user that sends the welcome message",
+            suggestions: [
+              "lain"
+            ]
+          }
+        ]
+      },
       %{
         group: :email,
         type: :group,