This is a Discourse 1.2 feature.
Phase 1: select emoji style
In the admin, site settings, offer a choice of emoji sets
Choices are: Apple/International, Google, Twitter, Emoji One and None.
Does not need to be pretty and graphical selector, can be a drop-down with text choices for now.
Phase 2: add per-site custom emoji
In the admin, customize, Emoji offer a way to add a custom emoji for your Discourse instance:
define the new emoji word used, e.g “fired”, which will equate to
add the image and associate it with the emoji word, e.g.
provide a basic UI for browsing, editing, and adding these custom emoji. Can be admin only for initial release.