Draft: feat: Purge public chat history older than month

I think the feature is useful, I am in a lot of public groups, and it makes everything slower (like changing chat tab). This makes it a bit better. Let me know if I should put in more work to finalize this or if you don't think the feature is worth it. I would argue it is for us users of computers considered "old" (in my case, an i7-2600K with a SATA3 SSD).

Maybe it should be an (optional) auto-purge instead of a manual action.