Emoji improvements

[plexus]

Thanks to @dawranliou and @oxalorg we already have some pretty solid emoji support, but there is always room for improvement :)

As authorative source I would like to eventually switch to https://github.com/iamcal/emoji-data

As per Emojipedia:

Slack permits a set of :shortcodes: to enter emojis on the platform. Typing an emoji short code such as ✨ inserts the ✨ Sparkles emoji.

This is stored in plain text, so copying-and-pasting a block of text which includes emojis added via shortcode will display the :shortcode: instead of the emoji character. Emoji shortcodes on Slack are derived from this repository, and do not necessarily match the same codes used on platforms such as Github.

This will make sure we have the exact some shortcodes that slack uses, and that we correctly handle multi-codepoint emoji that use ZWJ or skin tone selectors.

The main fields we care about are unified and short_names, unified can be rendered with HTML entities, e.g.

1F645-200D-2642-FE0F -> 🙅‍♂️

The second potential improvement is to render emojis as images. It pains me as a unicode fan to even suggest this, but this is what all major platforms do, since it's the only way to get consistent rendering (and thus consistent communication).

Except that Slack is not consistent themselves, they used to use the Apple emoji set, but that one is copyrighted, so now they use the Apple emoji set only when accessed from Apple devices. Otherwise they use Google's emojis.

https://blog.emojipedia.org/slack-overhauls-emoji-support/

I don't think we necessarily need to go that far, I'd be fine with rendering Google's emojis on all platforms.