Replace cleanslate with shadow dom

[glacambre]

As mentionned by @bovine3dom in #1134

[glacambre]

Looks like we'll have to wait a year if we don't want to break ESR compatibility or maintain two versions of the command line: https://en.wikipedia.org/wiki/Firefox_version_history#Firefox_60.3esr

[bovine3dom]

Oh wow, that would be a pain. We can just set min version to something so that ESR will at least get an old working version. Let's see how many ESR users we have and make a call.

[glacambre]

https://addons.mozilla.org/en-US/firefox/addon/tridactyl-vim/statistics/usage/applications/?last=30
Looks like about 500 users are using ESR.

[glacambre]

We can also use the all css property in order to reset an element's CSS. Shadow DOM is better though.

[bovine3dom]

Looks like we'll have to wait a year if we don't want to break ESR compatibility or maintain two versions of the command line

I have been consciously only supporting Firefox stable (except with backported security patches for ESR) for a while now, so this actually probably wasn't blocked for very long at all.

[bovine3dom]

Related to #112

[bovine3dom]

Vimium uses the shadow DOM and that apparently makes it more resistant to fingerprinting - philc/vimium#2399

[glacambre]

In the case of Tridactyl, it wouldn't. Frames are still accessible through window.frames and you can access their url (which contains a user-specific add-on installation ID). Changing the name of the HTML document of the command line to something like "index.html" (which is what I use in Firenvim) might help a little but not much.