CE-1103 Show a notification if a given wikia is available in the user's native language

https://wikia-inc.atlassian.net/browse/CE-1103

This is an extension that shows a notification if a wikia visited by a user has a sibling wikia in the user's native language. The check for the target language is based on a value of the *Geo* cookie. Then, there is a request sent via Nirvana to check if there is a record in the *city_list* table with a value of *city_url* matching the converted url (e.g. *http://community.wikia.com* is converted to *http://ja.community.wikia.com).

Currently, the only language that is supported is Japanese. However, adding a new language is a piece of cake and takes only adding an entry to an array in the JS module.

CC: @Grunny, @macbre, @nandy-andy 
Same here: `$this->wg->ExternalSharedDB`.