Add target language to data model

This should probably be behind the more_languages flag. We need a way to add the target language, so we can provide a view for heralds which languages the talks will be translated to.

Maybe make the current assignment view for logged-in users only and have the public view be something ultra-compact?

Cf. feedback pad:

• API / Web view: An easy way for heralds to see what languages will be available
  • The overview in “my assignements” was well structured. Maybe that can be done for available languages on THAT talk too?
• Related: Sometimes the translators have multiple common target languages, so explicitly setting one on the talk would help. Bonus points for auto setting when there is only one common language.