- Removed `MessagesProvider` and replaced it with a new
  `LocalizationProvider` component.

  The `LocalizationProvider` component takes one prop: `messages`.  It
  should be an iterable of `MessageContext` instances in order of user's
  preferred languages.  This iterable will be used by `Localization` to
  format translations.  If a translation is missing in one language,
  `Localization` will fall back to the next locale.

- The relevant `MessageContext` is now cached in `Localized`'s state.