Convert some settings to a model field for each contact form.

It will be useful to have different reply-to, sender, and send copy settings for each contact form so that:

```
CONTACT_PLUS_FROM_EMAIL
CONTACT_PLUS_REPLY_EMAIL_LABEL
CONTACT_PLUS_SEND_COPY_TO_REPLY_EMAIL
```

should be converted to a model fied changing these settings into default values for the corresponding field.

