Message formatting

This relies on chatroom being mostly implemented.
This will likely need to be split into subissues.

Messages are more than text: we should have some basic formatting capabilities, probably using a Markdown syntax.
- Bold, italic, underline?
- pinging people with '@'
- Formatting the person's name to make it clearly distinct from the message

Start by reviewing the chatroom implementation, which the issue says must be mostly complete before this work begins. Break the broad formatting request into separate capabilities such as Markdown styling, @-mentions, and distinct names; define the scope and completion criteria for each before implementation.

Written by the indexing model from the issue text.