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/forgot-password.html

@@ -14,7 +14,7 @@ Create a new password
 
     <p>We’ll send you an email to create a new password.</p>
 
-    <form autocomplete="off" method="post">
+    <form autocomplete="off" method="post" novalidate>
       {{ textbox(form.email_address, safe_error_message=True) }}
       {{ page_footer("Send email") }}
     </form>