Enhance external sites app

- adding OCP\JSON::checkAppEnabled('external') where needed
- in case a requested site is unknown we redirect to the default page
- fix X-Frame-Option header (addHeader method is so wrong)
- adding notes on iframe and http/https
- getting rid of usage of classpath and rename class OC_External to \OCA\External\External
- re-enabling save message
- fix javascript coding style

refs #1638 
refs https://github.com/owncloud/core/issues/9626
"Further more" => "Furthermore"

@tomneedham @Xenopathic native speaker check please.