This will generate emojis of this style while allowing the user to select the background colors, text colors and text.
To use these in slack you will have to size them down appropriately using a separate online tool for now. Search for resize animated gif in your favorite search engine.
This code is licensed MIT
Includes p5.js which is licensed LGPL.
Includes gif.js which is licensed under MIT.
Includes Ubuntu font which is licensed under the Ubuntu font license