feat: Add confirmation message after application submission

Sends a confirmation message to the user once an application
is submitted, thanking them and advising them to wait for
moderator review. Enhances user experience by providing clarity
and reassurance about the submission process.

src/matrix_applicationbot/classes/bot.py

@@ -350,6 +350,15 @@ class ApplicationBot:
                     },
                 )
 
+                await self.client.room_send(
+                    room.room_id,
+                    "m.room.message",
+                    {
+                        "msgtype": "m.text",
+                        "body": "Thank you for submitting your application. Please wait for a moderator to review it.",
+                    },
+                )
+
         await self.relay_message(room, sender, event)
 
     async def approve_or_reject_application(self, room, sender, command, approved):