input: Write a function to send an email using Celery and Flask-Mail.
  assert send_email('John Doe', 'johndoe@example.com', 'Test Subject', 'This is a test message.')==None
  assert send_email('Jane Doe', 'janedoe@example.com', 'Hello', 'Hello Jane!')==None
output: from flask import Flask
  from flask_mail import Mail, Message
  from celery import Celery
  
  app = Flask(__name__)
  app.config.update(
      MAIL_SERVER='smtp.example.com',
      MAIL_PORT=587,
      MAIL_USE_TLS=True,
      MAIL_USE_SSL=False,
      MAIL_USERNAME='your-email@example.com',
      MAIL_PASSWORD='your-password'
  )
  
  mail = Mail(app)
  
  celery = Celery(app.name, broker='redis://localhost:6379/0')
  
  @celery.task
  def send_async_email(msg):
      with app.app_context():
          mail.send(msg)
  
  def send_email(recipient_name, recipient_email, subject, body):
      msg = Message(subject, 
                    sender="your-email@example.com", 
                    recipients=[recipient_email])
      msg.body = f"Hello {recipient_name},\n\n{body}"
      send_async_email.delay(msg)