Mention log/message retention period on https://jmp.chat

Privacy is a concern for many of JMP's users.

A common question (one that I've asked at least twice) is how long the messages are retained.

The current answer is that metadata is stored on the upstream service for 30 days, message contents for up to 3 days.

This should be made explicit in the FAQ, even if it's going to change (ie, a reported upcoming API change resulting in no storage on the upstream service, so additional storage is planned directly on JMP services for debugging purposes).