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

首先阅读 issue #703 和链接的评论，以了解计划中的 HTTPS 和 persistent-storage 迁移。然后追踪现有的 HTTP offline-storage 行为；当用户得到通知且其离线数据在 HTTPS 迁移后仍然保留时，这项工作就完成了。

由索引模型根据 Issue 内容生成。