Contact form submit button should read "Send message"

The mail function of the contact form is located in the form's submit handler, which can be overriden. Therefore it is not sure if an e-mail will be sent or something else will happen when submitting the form. Also, the current text ("Send e-mail") may be confusing, since visitors will need to enter their own e-mail address. This patch converts the button's text to "Send message".