Turn off HTML5 validation on registration form

We do our own server-side validation of things like email address. We
don’t want the browser also trying to do it based on the input type of a
form field. It’s bad because the browser validation message comes up as
a nasty little tooltip under the field.

app/templates/views/register.html

@@ -11,7 +11,7 @@ Create an account
 <div class="grid-row">
   <div class="column-two-thirds">
     <h1 class="heading-large">Create an account</h1>
-    <form method="post" autocomplete="nope">
+    <form method="post" autocomplete="nope" novalidate>
       {{ textbox(form.name, width='3-4') }}
       {{ textbox(form.email_address, hint="Must be from a central government organisation", width='3-4', safe_error_message=True) }}
       {{ textbox(form.mobile_number, width='3-4', hint='We’ll send you a security code by text message') }}