Feature Request: allow adding reaction to the events on channel

For example:
I would like to add some reaction emoticon for the event that somebody has entered or left the channel.

The issue names no files, tests, or entry points. Start by locating channel event handling and existing reaction support, then determine how join and leave events are represented and where reactions must be added. Done means the requested reactions work for both event types and are covered by appropriate tests.

Written by the indexing model from the issue text.