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/send-test.html

@@ -14,7 +14,7 @@
     {{ page_title }}
   </h1>
 
-  <form method="post" class="js-stick-at-top-when-scrolling" data-module="autofocus" autocomplete="off">
+  <form method="post" class="js-stick-at-top-when-scrolling" data-module="autofocus" autocomplete="off" novalidate>
     <div class="grid-row">
       <div class="column-two-thirds">
         {{ textbox(