[Feature Request] Provide option for read receipts in single and group chats

One feature that Keybase is missing that exists on many other messaging platforms is the option to have read receipts. These would not be forced, they'd be opt-in. But they would be very helpful in many circumstances.

The issue names no files, tests, or entry points. Start by locating the chat implementation and preference handling for single and group conversations, then determine how an opt-in receipt can fit the existing messaging and encryption design. Done requires an agreed design and implementation covering both chat types.

Written by the indexing model from the issue text.