Move parsers into express-common module

This will allow endpoint modules to import them themselves, instead of
requiring server.js to pass them in.

src/express-common.js

@@ -0,0 +1,8 @@
+const express = require('express');
+
+// Instantiate parser middleware here with application-level size limits
+const jsonParser = express.json({ limit: '200mb' });
+const urlencodedParser = express.urlencoded({ extended: true, limit: '200mb' });
+
+module.exports = {jsonParser, urlencodedParser};
+