We weren't using this very much and introduces a dependency between
discourse-common and discourse which makes moving to yarn workspaces
more difficult.
In the future we might user ember-addons properly but for now it's
easier to move the code into discourse-common.
Note the old folder is still there because at least one plugin was still
requiring the old files. It will be removed in the future.
Robin Ward
Jarek Radosz
Joffrey JAFFEUX