Add autocomplete to email address on register form

GOV.UK Design System recommends:
> You should also set the autocomplete attribute to email. This lets
> browsers autofill the email address on a user’s behalf if they’ve
> entered it previously.

Only doing this on the register and sign in forms because it’s unlikely
to be helpful where a user is trying to enter someone else’s email
address.

tests/app/main/views/test_manage_users.py

@@ -387,6 +387,16 @@ def test_invite_user_allows_to_choose_auth(
     assert sms_auth_radio_button.has_attr("disabled") is False
 
 
+def test_invite_user_has_correct_email_field(
+    client_request,
+    mock_get_users_by_service,
+    mock_get_template_folders,
+):
+    email_field = client_request.get('main.invite_user', service_id=SERVICE_ONE_ID).select_one('#email_address')
+    assert email_field['spellcheck'] == 'false'
+    assert 'autocomplete' not in email_field
+
+
 def test_should_not_show_page_for_non_team_member(
     client_request,
     mock_get_users_by_service,