Implement a private/read-only mode

[kelson42]

This is a way to protect the privacy of the user.

This is a mode which should not write anything on the storage.

This could be a feature to be activated via the UI, like on any browser.

But we should also consider to maybe force activation via a file on the fs (for portable version) via .private file.

We should also discuss if such a feature should simply forbid to download ZIM files? Or would that be something different, like a "read-only" mode.

Follower of #1166