DEV: Make `discourse-common` an Ember addon. (PR #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.

GitHub

This is pointless now!

I noticed discourse had MIT in our package.json so I took the opportunity to correct it.

I see it generates a warning:

warning package.json: License should be a valid SPDX license expression

Looking at the list (https://spdx.org/licenses/), it apparently should be:

  "license": "GPL-2.0-only",

(or GPL-2.0-or-later)

Weird I literally got that string from a list of valid values! I guess it was wrong or outdated.