We rely on yarn workspaces so we don't want people using npm in the repo by accident.
Also updated the required node version to 12+.
~~Not sure about the min yarn version – the latest one could be missing in various CI-like envs, so I might change it yet.~~
Downgraded yarn to ">= 1.21.1" (the oldest of "current" versions, tagged "legacy")
* DEV: Use Ember CLI middleware to decorate the index template
Previously we'd do this on the client side which did not support our
full plugin API. Now requests for the index template will contact the
dev server for a bootstrap.json and apply it to the current template.
* FIX: Allows logins in development mode for Ember CLI
