Adds a temporary user_preference key, DARK_LANGUAGE_KEY, to hold the user's dark lang preference.
This preference key is deleted when ?clear-lang is placed in query params.
To mark a language as dark-launched, add it to the DARK_LANGUAGES
django conf setting. To activate a dark-launched language, set he
query parameter `preview-lang` to the language code on any url.
[LMS-2045]
[LMS-2077]
[LMS-2076]