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/support/problem.html

@@ -20,7 +20,7 @@
             page to see if there are any known issues with GOV.UK Notify.
           </p>
         </div>
-        <form method="post">
+        <form method="post" autocomplete="off" novalidate>
             {{ textbox(form.feedback, width='1-1', hint='', rows=10) }}
             {% if not current_user.is_authenticated %}
               {{ textbox(form.name, width='1-1') }}

app/templates/views/support/question.html

@@ -13,7 +13,7 @@
     </h1>
     <div class="grid-row">
       <div class="column-two-thirds">
-        <form method="post">
+        <form method="post" autocomplete="off" novalidate>
             {{ textbox(form.feedback, width='1-1', hint='', rows=10) }}
             {% if not current_user.is_authenticated %}
               <h3 class="heading-medium">Do you want a reply?</h3>