Fixes #14768

* Add emojis:generate_borders Rake task

* Address review

* Border all dark emoji

* Combine stroke with filter to reduce artifacting

* Cleanup Camera with Flash

* Add stroke-linejoin="round"

The previous filter and tweaks were effectively a poor imitation of it.
There are no artifacts for any dark emoji now!

* Set stroke-width using property

This fixes old versions of Firefox.

* Store emoji in string instead of array

* Use separate arguments for each path segment

* Remove "background: black;"

* Update Twemoji to 12.1.5

SVGs from Twemoji repo https://github.com/twitter/twemoji

* Update emoji_map.json to Unicode 12

A new rake task emojis:generate downloads a full list of valid
unicode sequences from unicode.org and checks it against existing
Twemoji files, finally generating a map from each sequence to the
existing file (e.g. when there's multiple ways an emoji can be
expressed). The map is dumped into app/javascript/mastodon/emoji_map.json

That file is loaded by emojione_light.js (now a misnomer) which
decorates it further with shortcodes taken from emoji-mart's index.