Upgrade django_countries to get more up-to-date countries list (incl South Sudan)

Upgraded django_countries from 1.0.5 to 2.1.2 so that we get a more up-to-date list of countries. However, the coding interface has changed slightly - I've fixed the askbot code accordingly, but was unable to test the bits that aren't used in KnowledgePoint project:

* templatetags/extra_filters_jinja.py's country_display_name() (should be OK)
* templatetags/extra_filters_jinja.py's country_flag_url() (you'll probably need to provide configuration for this to get the exact correct URL back (see Customize the flag URL section at https://pypi.python.org/pypi/django-countries/2.1.2)