Require email address for reporting problems

If you report a problem we want to be able to get back to you to find
out more information, or to update you on the status of a fix. So it
shouldn’t be possible to report a problem without providing an email
address.

This commit makes `email_address` a required field when `ticket_type` is
problem.

This requires a bit of fiddling with the tests which weren’t expecting
to have to provide an email address. So the tests now either:
- pass an email address
- check for an error when they don’t pass an email address

app/templates/views/support/problem.html

@@ -23,8 +23,6 @@
         <form method="post">
             {{ 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>
-              <p>Leave your details below if you'd like a response.</p>
               {{ textbox(form.name, width='1-1') }}
               {{ textbox(form.email_address, width='1-1') }}
             {% else %}