chore: add proxy backend (#3126)

* feat(backends): add proxy backend

* feat: add proxy server initial commit

* fix: move from joi to @hapi/joi

* test: add joi validation tests

* feat: proxy server initial implementations

* test: add tests, fix build

* chore: update yarn.lock

* build: fix develop command

* fix(back-proxy): fix bugs

* test(backend-proxy): add cypress tests

* chore: cleanup

* chore: support node 10

* chore: code cleanup

* chore: run cypress on ubuntu 16.04

* test(e2e): fix proxy backend cypress tests

* chore: don't start proxy server on yarn develop

package.json

@@ -2,7 +2,7 @@
   "scripts": {
     "bootstrap": "lerna bootstrap",
     "start": "run-s clean bootstrap build:esm develop",
-    "develop": "lerna run develop --parallel",
+    "develop": "lerna run develop --parallel --ignore netlify-cms-proxy-server",
     "build": "run-s clean build:esm build:lerna",
     "build:lerna": "lerna run build",
     "build:esm": "lerna run build:esm",