Remove HTML5 validation

This commit:
- uses WTForms email fields wherever we collect email addresses
- makes sure we don’t let the default HTML5 form validation get trigged
  (using `novalidate`[1])

We don’t want to use the default validation because:
- it looks out of place
- it’s not well supported by assistive technology

1. https://developer.mozilla.org/en-US/docs/Web/HTML/Element/form#attr-novalidate

app/templates/views/user-profile/change.html

@@ -17,7 +17,7 @@
       </p>
     {% endif %}
     <div class="column-three-quarters">
-      <form method="post">
+      <form method="post" autocomplete="off" novalidate>
         {{ textbox(form_field, safe_error_message=True) }}
         {{ page_footer(
           'Save',