Add Support for Rich Text Formatting

[undercase]

Users will want some way to include formatted text-links, bolded text, italicized text, etc. We should extend the application with some standard formatting system like markdown or HTML.

[andrewxhill]

just flagging this as something we'll want consider a generic solution that can be reused / incorporated by other apps with chat.

gut says a json schema is going to be flexible for validating on multiple clients, parsing, extending. also with node level schema validation, you can enforce other apps tapping into a chat thread use the same rules and so can be viewed by other clients.

been going iteratively deeper on slate's schema for note based content, https://docs.slatejs.org/slate-core/schema