https migration with no data loss

HTTPS support is needed to be able to "add option to make offline storage persistent":

https://github.com/freeCodeCamp/devdocs/issues/703#issuecomment-340320802

> Before implementing persistent storage, I want to force HTTPS, so that the feature is available to everyone. However, forcing HTTPS will nuke all HTTP users' offline data (but not their settings—like you said), so before doing that I need to implement a few things to let the users know and ease the transition.

This ticket is to track that migration

Inizia leggendo l’issue #703 e il commento collegato per comprendere la transizione pianificata a HTTPS e persistent-storage. Poi traccia il comportamento esistente di HTTP offline-storage; il lavoro è completato quando gli utenti sono informati e i loro dati offline sopravvivono alla migrazione HTTPS.

Scritto dal modello di indicizzazione a partire dal testo della issue.