Hi,
this is a tricky one. From my experience, this should go through a plugin. You’ll have to add additional button to post-menu widget, and you’ll have to add a migration to db to store your “made you laugh” reactions.
You could use discourse solved plugin for inspiration:
This plugin also adds additional button to post menu and adds functionality that is stored in db.
Make sure you read the developers guide for building plugins:
Good luck.