Add header-checking function (#9880)

* Add basic header-checking function

* Add Slack notification logic

* Modify Netlify Functions setup

* Rework function logic

* Add missing dependencies

* Remove package-lock.json from Git

* Separate header checking function

* Isolate env check into a separate function

package.json

@@ -0,0 +1,10 @@
+{
+  "private": true,
+  "devDependencies": {
+    "@babel/core": "^7.0.1",
+    "@slack/client": "^4.4.0",
+    "babel-loader": "^8.0.2",
+    "netlify-lambda": "^0.4.0",
+    "node-fetch": "^2.2.0"
+  }
+}