This modifies the lang-pref django app to:
a) Use the current value of the 'edx-language-preference' cookie to set the
users Accept-Language header on an incoming request.
b) At the end of the request, update the 'edx-language-preference' cookie
to reflect the users current Language Preference choice, if any.
[LEARNER-542]
This feature is not well-documented, so it is unclear how to setup Elasticsearch properly to make it work. The next person who needs to work on this feature will need to make it work and, ideally, document it.
