Adding support for crossdomain middleware

User can edit full value of cross domain policies
I image it's easy for people to do a typo in the value for this attribute. Can we add some validation to `validate_proposal_after_save` in swift_service.rb? Maybe just checking that it's valid xml (when enclosed inside `<cross-domain-policy>`)?

(ideally we would check with the DTD, but I don't know if it's available on the system and we can't rely on downloading it)