Adjust contact form

Feedback from Peter Kraume:

Hey guys, great work to get a contact form for t3o. I have some wishes:

• please set the ReplyTo address to the email address of the sender
• add a mandatory subject field to the form and make it the subject of the mail to info@typo3.org
• don't add the name of the sender as the name of the recipient (info@typo3.org).

Keep up the good work!