suggestion -- pinned (or starred) messages

Hello -- thanks for all the recent feature updates, I teach keybase in college classroom and community settings and folks are using it to replace encrypted email (much more readily scalable than PGP email).

For this to be feasible, a bit more message management would really help. Could keybase implement pinned messages (which appear at the top of a chat for the whole team, in a hideable pane, similar to the separation of teams/big teams)? Also/alternatively allow the client to "star" messages (similar to starred email messages in Thunderbird; allowing a user to filter for important messages, e.g. for those that represent outstanding tasks).

Thank you! (this is a more detailed suggestion first made w/ #3138)

No files, tests, or entry points are named; start by reviewing the related suggestion in #3138 and the existing chat product context. Clarify whether team-wide pinned messages, personal starred messages, or both are in scope, then define the client and service behavior that would constitute completion.

Written by the indexing model from the issue text.