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

Start by reading issue #703 and the linked comment to understand the planned HTTPS and persistent-storage transition. Then trace the existing HTTP offline-storage behavior; the work is done when users are informed and their offline data survives the HTTPS migration.

Written by the indexing model from the issue text.