- 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
- show start saving message
- validate url to start with http:// or https://
Merge pull request #1851 from owncloud/backport-1850-stable6

- adding OCP\JSON::checkAppEnabled('external') where needed