input: Write a function to create a Django project and an app within it.
  assert create_django_project_and_app('myproject', 'myapp') == ('Project myproject created.', 'App myapp created.')
output: import os
  import subprocess
  
  def create_django_project_and_app(project_name, app_name):
      # Create the Django project
      subprocess.run(['django-admin', 'startproject', project_name], check=True)
      
      # Change directory to the project directory
      os.chdir(project_name)
      
      # Create the Django app within the project
      subprocess.run(['python', 'manage.py', 'startapp', app_name], check=True)
      
      return (f'Project {project_name} created.', f'App {app_name} created.')