This makes a small improvement to 'cold cache' ember-cli build times, and a large improvement to 'warm cache' build times
The ember-auto-import update means that vendor is now split into multiple files for efficiency. These are named `chunk.*`, and should be included immediately after the `vendor.js` file. This commit also updates the rails app to render script tags for these chunks
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")
