validate email addresses in one-off flow

previously we were just using the wtforms builtin email validator,
which is much more relaxed than our own one. It'd catch bad emails when
POSTing to the API, resulting in an ugly error message. It's easy work
to make sure we validate email addresses as soon as they're entered.

tests/app/main/test_placeholder_form.py

@@ -22,6 +22,7 @@ def test_form_class_not_mutated(app_):
 
     (False, 'email address', '', 'Can’t be empty'),
     (False, 'email address', '12345', 'Enter a valid email address'),
+    (False, 'email address', '“bad”@email-address.com', 'Enter a valid email address'),
     (False, 'email address', 'test@example.com', None),
     (False, 'email address', 'test@example.gov.uk', None),