Update example.com to match the mail host if user does not update it
Ideally, we should prevent side effects, but this is something that confuses users a lot.
The main purpose of Django Sites framework is to have the right Title in Hyperkitty right now. Other than that, we don't depend on sites framework too much right now, but we do plan to depend on it to generate full URLs to approve subscriptins and such in future.
The idea is to update the Site object associated with the domain when it is created if and only if the Site is example.com
and leave it as-is otherwise. This should atleast confuse users much less when they create the domain and still see example.com.
Note that we aren't going to create a new Site, but simply update the Site chosen in the create domain form if the domain is example.com
.