Display email address that invitee will be registered with.

Also add flash message for users who already have an account.

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

@@ -11,6 +11,7 @@ Create an account – GOV.UK Notify
 <div class="grid-row">
   <div class="column-two-thirds">
     <h1 class="heading-large">Create an account</h1>
+    <p>Your account will be created with this email: {{email_address}}</p>
     <form method="post" autocomplete="nope">
       {{ textbox(form.name, width='3-4') }}
       {{ textbox(form.mobile_number, width='3-4') }}