Add functionality to check & set opt-in flag from code, also for anons

Adds anew mmv.Config class to centralize localStorage handling issues,
adds AJAX option setting, adds getting/setting opt-in flag from
localstorage.

Change-Id: I25762ccb8c9bccd694c30a6f1c5d3c8611a5ecbc
Mingle: https://wikimedia.mingle.thoughtworks.com/projects/multimedia/cards/703
Mingle: https://wikimedia.mingle.thoughtworks.com/projects/multimedia/cards/704
Merge "Add functionality to check & set opt-in flag from code, also for anons"