Add .typesettingLanguage view modifier to post/comment content

[Sjmarf]

We attach .typesettinglanguage to the Text view in post/comment content, passing in the language for that post/comment.

This makes Text render it in the ideal way for that language. If this isn't set, it uses the language of the app UI as the default typesetting language. This might become a problem if we offer the app in languages where line spacing is different (e.g. Urdu, maybe some others), because that taller line spacing will be applied to all text in the app, including post/comment text in English and other languages.