Attempts to resolve faulty validation when copying a project.

*Fix coming from [this FogBugz](http://manage.dimagi.com/default.asp?108982#631214) issue, which details how a 500 error was raised by somebody entering the same domain name while trying to copy an app.* 

The original error was thrown from line 282, where `form.clean_domain_name()` was being explicitly called.  There's no need to do that, as that method is already called once when `form.is_valid()` is called on 281  (Instead of calling again we can just drop in the cleaned value.

So, we won't get a `ValidationError` outside of Django's normal process, and the error is properly being raised to the user, but that doesn't really explain how the error actually happened -- if `clean_domain_name()` raised this `ValidationError` on 282, why didn't it catch it when it was run by `is_valid()` on 281?  **I could use some help brainstorming this -- can anybody imagine any reason why this would happen?**  This code has been explicitly calling `clean_domain_name()` since 2012, and presumably this is the first time this has come up?