Add Mirage-toggling via environment variable (#5899)

I’m finding myself having to revert my change to this
variable when I switch branches, so this would let me
affect the variable without code changes.

ui/config/environment.js

@@ -1,6 +1,10 @@
 /* eslint-env node */
 
-const USE_MIRAGE = true;
+let USE_MIRAGE = true;
+
+if (process.env.USE_MIRAGE) {
+  USE_MIRAGE = process.env.USE_MIRAGE == 'true';
+}
 
 module.exports = function(environment) {
   var ENV = {