feat(core): auto detect proxy server on load (#3195)

* feat: auto detect proxy server on load

* fix: opt-in for auto proxy server detection

dev-test/backends/proxy/config.yml

@@ -1,11 +1,15 @@
 backend:
-  name: proxy
+  name: github
   branch: master
-  proxy_url: http://localhost:8082/api/v1
+  repo: owner/repo
 
 publish_mode: editorial_workflow
 media_folder: static/media
 public_folder: /media
+
+local_backend:
+  url: http://localhost:8082/api/v1
+
 collections:
   - name: posts
     label: Posts