refactor(jQuery): Remove $.ajax, use async/await, move custom paths t…

…o their own test

- I wanted to remove $.ajax, so replaced it with fetch
- Used async/await hopefully makes the tests easier to follow
- We were testing custom paths using `posts` model in every test, moved testing of this behaviour into it's own test and removed from all other tests.

package.json

@@ -68,6 +68,7 @@
     "ember-disable-prototype-extensions": "^1.1.2",
     "ember-export-application-global": "^2.0.0",
     "ember-fastboot-addon-tests": "^0.4.0",
+    "ember-fetch": "^3.4.3",
     "ember-load-initializers": "^1.0.0",
     "ember-resolver": "^4.0.0",
     "ember-sinon": "0.5.1",