[F.Rq.] Syntax highlighting for forum markup

[YoshiRulz]

I'm thinking JS could replace the textarea with a richer UI, eventually a MediaWiki-style editor, but initially it could be as simple as underlining BBCode tags in red or grey depending on whether they're in the list of valid tags. I've mocked that up here:

[nattthebear]

This could be pretty complex to do, especially if we want to avoid big libraries.