Validate email address in checkout form.

Adds validation for the email address.

I've added a very simple regex check for the email address, since the more complex checks usually do not allow valid but exotic email addresses.
The simple check is that the address contains exactly one `@`, which is not followed or preceded by a `.`. It should also contain at least one `.` in the domain part of the address.

@lukasgraf could you take a look?