Language Selection

Fixes #144. This adds a language selection dropdown to the footer which allows the user to change their language which will result in changing the site content. This will also make it clear that #160 is not an issue.
No the Mezzanine URLs don't use a the `i18n_patterns` so the URL won't contain the language code. Instead this change will set the `django_language` cookie which will be picked up by the `LocaleMiddleware` setting the `request.LANGUAGE_CODE` later used by the `LocaleSiteMiddleware` which changes the Mezzanine site based on the selected language. It preserves the URLs without the locale prefixes but still allows switching the language. It does make it hard to see that it has changed if you don't have different content on the English and Chinese versions of the page.