Reformat message tokens for Discord output

Tokens formatted with __ (underline) or _ (italic) break basic bare text search in Discord. * (italic) and ** (bold) are fine. Fix DiscordOutputAdapter to use the */**, and try to find an underline equivalent. There is no "official" Markdown underline (https://www.markdownguide.org/basic-syntax/), so maybe Discord implemented one?

Try to address TODO about the markdown builders at the same time.