Put hidden email field on register from invite page

Password managers will try to guess what they should save as a username
by looking at the fields on the page where you set up your password.

When registering from an invite the email address (what we use as a
username) is predefined, and only shown on the page as text, not an
input.

This commit also adds a hidden input field for password managers to pick
up.

Adapted from: https://github.com/UKGovernmentBEIS/beis-opss-psd/blob/master/app/views/users/complete_registration.html.erb#L29-L36

app/templates/views/register-from-invite.html

@@ -17,6 +17,9 @@ Create an account
       <span class="nowrap">{{invited_user.email_address}}</span>
     </p>
     {% call form_wrapper() %}
+      <div class="visually-hidden">
+        <input type="email" name="username" id="username" value="{{ invited_user.email_address }}" disabled="disabled" tabindex="-1" aria-hidden="true" autocomplete="username" />
+      </div>
       {{ textbox(form.name, width='3-4') }}
       {% if invited_user.auth_type == 'sms_auth' %}
         <div class="extra-tracking">