input: Write a function to create a web application using Flask that takes user input and generates an HTML page displaying the input. The function should return the Flask app object.
  assert create_webapp() is not None
  assert create_webapp().testing is True
output: 
  from flask import Flask, request, render_template_string
  
  def create_webapp():
      app = Flask(__name__)
  
      @app.route('/', methods=['GET', 'POST'])
      def index():
          if request.method == 'POST':
              user_input = request.form['user_input']
              return render_template_string('<h1>{{ user_input }}</h1>', user_input=user_input)
          return '''
              <form method="post">
                  <input type="text" name="user_input">
                  <input type="submit">
              </form>
          '''
  
      return app
  