Fix and extend the in-place translation facility for local_settings.py

[zmousm]

• Add djnro.lldict.LazyLangDict, a dict-like object that
  • lazily filters keys by settings.LANGUAGES and
  • returns an arbitrary value in unicode() context.
• Use said object to wrap in-place translation in local_settings.py
• Extend in-place translation to REALM_COUNTRIES, an iterable that provides
  • choices for edumanage.models.Realm.country, where Django seeks a text value for the label,
  • the COUNTRY_NAME context variable, which is used e.g. in 'participants' template, where in-place translation is desirable. The new dict wrapper object accommodates both such needs.