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/api/whitelist.html

@@ -42,7 +42,7 @@
     are included in the whitelist automatically.
   </p>
 
-  <form method="post">
+  <form method="post" autocomplete="off" novalidate>
 
     <div class="grid-row">
       <div class="column-two-thirds">