Only make URLs relative if its not external

https://github.com/retlehs/roots/commit/d611a219a52f94ea8452ef4a15185c775c32abe7
Use SERVER_NAME to avoid parsing site_url()

https://github.com/retlehs/roots/commit/9ac6bc698ebce6b6f180b3827166848edfd2442b

site_url() was previously retrieved and parsed many times per page
before. This is cleaner and done for optimization purposes.