DEV: Make discourse-common an Ember addon. (#9578)

This is to help with the migration to Ember CLI. In the current running
version of Discourse everything should be the same as before, just with
a few extra files that are not used. However, using Ember CLI this can
be installed as an Ember addon.

Co-Authored-By: Jarek Radosz <jradosz@gmail.com>

package.json

@@ -4,7 +4,7 @@
   "main": "index.js",
   "repository": "git@github.com:discourse/discourse.git",
   "author": "Discourse",
-  "license": "MIT",
+  "license": "GPL-2.0-only",
   "dependencies": {
     "@fortawesome/fontawesome-free": "5.11.2",
     "@popperjs/core": "v2.0.6",