Autofill/tokenize usernames from /chat-names

One thing that is really nice in Slack (especially in Slack's that aren't bridged with IRC) is being able to start typing and autofill a nickname of someone you want to mention.

It would be great if the [/chat-names](https://indieweb.org/chat-names) were logged and we had a javascript autocomplete UI that allowed you to autocomplete a username. This would allow you to more easily log !tells and such functions when trying to mention someone.

Start by tracing how /chat-names are logged and where the chat message composer handles username input. Add a JavaScript autocomplete UI for those usernames, then verify that selecting a username supports mentions and !tells as described.

Written by the indexing model from the issue text.