Emoji: Replace :roll: with unicode 2 twemoji version

[Joen]

Steps to reproduce: write a post with the :roll: smiley inserted.

Notice that it maps to /wp-includes/images/smilies/rolleyes.png, which is a custom smiley we drew to match the Twemoji style. We did this because Twemoji version 1 did not have a "face with rolling eyes" emoji at the time.

But Unicode 8.0 does, and so does the new version of Twemoji: ​https://s.w.org/images/core/emoji/72x72/1f644.png

Suggested fix: keep the :roll: string, map it to the Twemoji image now, and retire the old smiley PNG file.

[pento]

In 37296:

Emoji: The :roll: smiley is now an emoji.

Unicode 8 added U+1F644, "Face with Rolling Eyes", which matches our :roll: smiley. Continuing our ever present quest to rid the world of legacy smilies, we now translate that smiley into its emoji form.

Sadly, :mrgreen: remains sorely under-represented in the Unicode standard, I'm inclined to think that we should buy a voting membership of the Unicode Consortium, in order to rectify that situation. (Personally, I'm hoping for a green skin tone modifier.)

Fixes #36365.