Channel domain validation does not allow localhost

The validate function of Channel/Channel.php should be improved:

• All characters (including international) that can be in a domain should be accepted (such as a-z, 0-9, period, and international characters)

• Characters that can't be in a domain name such as "/" should not be allowed

• An empty primary domain is also OK

Valid:
example.com
localhost
افغانستا.icom.museum
belgië.icom.museum

Invalid:
http://example.com