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

まず issue #703 とリンクされたコメントを読んで、計画されている HTTPS と persistent-storage への移行を理解します。次に、既存の HTTP offline-storage の動作を追跡します。ユーザーに通知され、オフラインデータが HTTPS 移行後も保持されれば、作業は完了です。

索引モデルが issue の本文から書いたものです。