Audit classes for nullibility

[Wevah]

This includes re-auditing RSHTMLMetadata to change things I marked nullable that can actually never be nil, which will allow removal of some unneeded if let/guard let statements in NetNewsWire. (For example, appleTouchIcons is always initialized with an array, so it will never actually be nil.)

[brentsimmons]

Let’s do this — but let’s wait till after NNW 5.0 ships, just to minimize code changes at this point.