[Feature Request] Read Receipts/Last Online

I would love to have read receipts and last online status on Keybase as an option. It can be very useful for certain types of groups.

The issue names no files, tests, or entry points. First define the intended read-receipt and last-online behavior, including how the option should work for groups, then identify the affected chat components and client platforms. Done requires an agreed design and implementation across the relevant clients and services, but the issue does not specify those details.

Written by the indexing model from the issue text.