input: Write a python function to create a Flask app that renders a template and handles form submissions.
  assert index() == render_template("index.html", message="Hello, World")
  assert request.method == "POST" and index() == render_template("index.html", message="Form Submitted")
output: from flask import Flask, render_template, request
  
  app = Flask(__name__)
  
  @app.route("/", methods=["GET", "POST"])
  def index():
      if request.method == "POST":
          return render_template("index.html", message="Form Submitted")
      return render_template("index.html", message="Hello, World")