Fix Pin.net.au payments and Thymeleaf exception when showing signup template

Two small fixes.

* One of the recent changes to the HTML renamed the submit button, which broke the Pin.net.au payment form. I've reverted that line.
* Thymeleaf template exception shown to user if session expired and browser POSTed to /makePayment. "signup" template is used without a signupForm object in this case (PersonalCloudController.java:649)