Emoji reactions should use the colourful variation-16 selector when available

For emoji in https://unicode.org/emoji/charts/emoji-variants.html, we should prefer sending the colourful variation unless we are very deliberately sending a black and white one. This is more correct than leaving it ambiguous by missing off the selector, where some platforms may assume you meant the black and white variant. See https://github.com/vector-im/riot-web/issues/13926.