Instead of having Ember CLI know which URLs to proxy or not, have it try
the URL with a special header
HTTP_X_DISCOURSE_EMBER_CLI. If present,
and Discourse thinks we should bootstrap the application, it will
instead stop rendering and return a HTTP HEAD with a response header
telling Ember CLI to bootstrap.
In other words, any time Rails would otherwise serve up the HTML for the Ember app, it stops and says “no, you do it.”