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-proxy-server/tsconfig.json

@@ -0,0 +1,19 @@
+{
+  "compilerOptions": {
+    "module": "commonjs",
+    "esModuleInterop": true,
+    "allowSyntheticDefaultImports": true,
+    "target": "ES2018",
+    "moduleResolution": "node",
+    "outDir": "dist",
+    "baseUrl": ".",
+    "allowJs": true,
+    "strict": true,
+    "noImplicitAny": true,
+    "noUnusedLocals": true,
+    "noUnusedParameters": true,
+    "sourceMap": true
+  },
+  "include": ["src/**/*"],
+  "exclude": ["src/**/*spec.ts"]
+}