Dialog demo: Update modal form demo

- Removes an invalid jquery.ui.button.js reference (button.js is loaded)
- Updates the email regex to use the one from the HTML5 spec
- Refactors the code to add the user on both button click and form submit
- Reset the form to its original state on submit
- Initialize the form with values that can be submitted immediately, better
for a demo
- Rename bValid to valid