Enabled Renovate to manage dependencies

- this adds Renovate to the repository to manage dependencies
- I've configured it with our `base` config first but we'll likely
  switch to `@tryghost:quietJS` to allow auto-merging once the build is
  stable

renovate.json

@@ -0,0 +1,5 @@
+{
+  "extends": [
+    "@tryghost:base"
+  ]
+}