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

packages/netlify-cms-backend-proxy/package.json

@@ -0,0 +1,29 @@
+{
+  "name": "netlify-cms-backend-proxy",
+  "description": "Proxy backend for Netlify CMS",
+  "version": "1.0.1",
+  "repository": "https://github.com/netlify/netlify-cms/tree/master/packages/netlify-cms-backend-proxy",
+  "bugs": "https://github.com/netlify/netlify-cms/issues",
+  "license": "MIT",
+  "module": "dist/esm/index.js",
+  "main": "dist/netlify-cms-backend-proxy.js",
+  "keywords": [
+    "netlify",
+    "netlify-cms",
+    "backend"
+  ],
+  "sideEffects": false,
+  "scripts": {
+    "develop": "yarn build:esm --watch",
+    "build": "cross-env NODE_ENV=production webpack",
+    "build:esm": "cross-env NODE_ENV=esm babel src --out-dir dist/esm --ignore \"**/__tests__\" --root-mode upward --extensions \".js,.jsx,.ts,.tsx\""
+  },
+  "peerDependencies": {
+    "@emotion/core": "^10.0.9",
+    "@emotion/styled": "^10.0.9",
+    "netlify-cms-lib-util": "^2.3.0",
+    "netlify-cms-ui-default": "^2.6.0",
+    "prop-types": "^15.7.2",
+    "react": "^16.8.4"
+  }
+}