Add markdown and emoji support for chat messages

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Chat messages should be able to support basic markdown features (italics, bold, code).

In addition, there should be an emoji feature with an emoji picker.

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I'm not aware of any very lightweight markdown parsers that are focused on providing just these minimal features, so we will roll our own parser. Should be interesting...

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Let's keep this on hold for now, there'll be a lot of work here.

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I'm looking at this for markdown: https://github.com/acdlite/react-remarkable

There's also a lot of emoji providers to choose from.