Fix production file links for some alt-domain configurations

Summary:
We sometimes call PhabricatorEnv::getProductionURI($file->getBestURI()) or
similar, but this may currently cause us to construct a URI like this:

http://domain.com/http://cdn-domain.com/file/data/xxx/yyy/name.jpg

Instead, if the provided URI has a domain already, leave it unmodified.

Test Plan: Attached a file to a task; got an email with a valid URI instead of
an invalid URI.

Reviewers: btrahan

Reviewed By: btrahan

CC: Makinde, aran, epriestley

Differential Revision: https://secure.phabricator.com/D1622