Add "Show Join/Leave in Chat" option

This small change adds an option to toggle the Join/Leave messages in public chat, similar to the original client option. <img width="504" alt="Screenshot 2024-05-11 at 7 56 28 PM" src="https://github.com/mierau/hotline/assets/868228/f0097116-c2da-4bff-86b7-5528b4cc391b"> <img width="444" alt="Screenshot 2024-05-11 at 7 57 00 PM" src="https://github.com/mierau/hotline/assets/868228/8642bfce-8e6a-405c-b05e-3e93249e5411">