improve: OAuth2 UI and test suite (via #5066)

* create `features` folder

* add base oauth2 server

* continue implementing OAuth tests

* WIP

* add password flow tests

* modify Password flow credential types

* remove query string credential type

* add test case for Authorization flow

* add specific Authorization value for Password flow test

* WIP

* fix linter issues

test/e2e-cypress/helpers/oauth2-server/index.js

@@ -0,0 +1,50 @@
+// from https://github.com/pedroetb/node-oauth2-server-example
+
+var Http = require("http")
+var path = require("path")
+var express = require("express")
+var bodyParser = require("body-parser")
+var oauthserver = require("oauth2-server")
+var cors = require("cors")
+
+var app = express()
+
+app.use(cors())
+
+app.use(bodyParser.urlencoded({ extended: true }))
+
+app.use(bodyParser.json())
+
+app.oauth = oauthserver({
+  model: require("./model.js"),
+  grants: ["password", "client_credentials", "implicit"],
+  debug: true
+})
+
+app.all("/oauth/token", app.oauth.grant())
+
+app.get("/swagger.yaml", function (req, res) {
+  res.sendFile(path.join(__dirname, "swagger.yaml"))
+})
+
+app.get("*", app.oauth.authorise(), function (req, res) {
+  res.send("Secret secrets are no fun, secret secrets hurt someone.")
+})
+
+app.use(app.oauth.errorHandler())
+
+function startServer() {
+  var httpServer = Http.createServer(app)
+  httpServer.listen("3231")
+
+  return function stopServer() {
+    httpServer.close()
+  }
+}
+
+module.exports = startServer
+
+if (require.main === module) {
+  // for debugging
+  startServer()
+}