Feature: automatic shortcodes for emoji

A simple feature, allows users to simply drop a folder of emoji named like "<shortcode>.png", and it creates the shortcode with the correct path, without having to touch config/emoji.txt. Users can configure which globs are used to find auto-shortcode emoji, which is by default set to ["/emoji/custom/**/*.png"]. Only the filename is used for the shortcode, which makes it easy to drop entire folders for organization and ease of management. If there are conflicting shortcodes, you can still specify it manually in config/emoji.txt to resolve the conflict, as that takes precedence.